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One and a half hours is one hour thirty minutes. The decimal part, 0.5, means half of an hour; half of sixty minutes is thirty minutes. Reading 1.5 hours as one hour fifty minutes changes the duration by twenty minutes before any later calculation has begun.
This guide shows you how to move between decimal hours, hours and minutes, and total minutes. You will also see why a time of day and a duration need different labels, how to use converted time in a rate question, and how to check an unfamiliar display before trusting its punctuation.
The examples are original teaching situations. Use the 2027 G1 Mathematics syllabus for official curriculum context and your school’s sequence for the work needed now. The underlying unit conversion is useful across subject levels and in everyday planning.
Contents
Sections 1–5
Sections 6–10
Sections 11–15
Sections 16–20
Sections 21–25
Sections 26–30
Sections 31–32
To convert decimal hours to minutes, multiply the whole duration by sixty. Thus 1.5 hours is ninety minutes, or one hour thirty minutes. Alternatively, keep the whole hour and multiply only the fractional part by sixty: 0.5 × 60 = 30. Both methods describe the same duration.
To convert one hour fifty minutes to decimal hours, divide the fifty minutes by sixty and add one. The result is 1 + 50/60 = 11/6 hours, approximately 1.8333 hours. The digits after a decimal point count tenths, hundredths and smaller fractions of an hour; they are not automatically a minute field.
Before entering a value into a formula, read its unit and format. A duration written 1:50 may mean one hour fifty minutes if the display uses hours and minutes. A duration written 1.50 h means one and a half hours. Labels decide the interpretation; a familiar-looking number cannot decide it alone.
In the decimal number 1.5, the five occupies the tenths place. It contributes five tenths of one unit. If the unit is hours, five tenths of an hour is 5/10 × 60 = 30 minutes. If the unit were metres, five tenths of a metre would instead be fifty centimetres.
The decimal system has not changed between those examples. What changed is the relationship between the named unit and the smaller unit. One metre contains one hundred centimetres; one hour contains sixty minutes. The same decimal fraction can therefore produce different smaller-unit numbers in different measurement systems.
This is why “move the decimal point” is a weak explanation for time conversion. There is no power of ten connecting hours with minutes. Begin from the known relationship, one hour equals sixty minutes, then multiply or divide according to the direction. Meaning gives you a method you can reconstruct.
The duration one hour forty minutes consists of a whole hour plus forty minutes. In total minutes it is sixty plus forty, or one hundred minutes. In decimal hours it is one plus forty sixtieths, or five thirds of an hour. These are different representations of one duration.
Writing 1.40 h gives one and forty hundredths of an hour. Forty hundredths of sixty minutes is twenty-four minutes, so 1.40 h represents eighty-four minutes. That is sixteen minutes shorter than one hour forty minutes. Appending a unit to an incorrectly interpreted decimal does not repair the difference.
For working that combines several durations, choose one common representation first. Total minutes are often easiest for addition. Decimal hours may be convenient in a kilometres-per-hour formula. Hours and minutes may be clearest for a timetable answer. The best format depends on the job, provided conversions preserve the duration.
If a duration is h hours, its total minutes are 60h. The numerical value grows because minutes are smaller units. For 2.35 hours, 60 × 2.35 = 141 minutes. Splitting that total into whole hours leaves two hours and twenty-one minutes. The thirty-five hundredths did not become thirty-five minutes.
If a duration is m minutes, its hours are m/60. The numerical value becomes smaller because an hour contains many minutes. For 141 minutes, 141/60 = 2.35 hours. Converting the answer back recovers the starting value, giving a straightforward check on both the direction and the arithmetic.
Keep the units beside the relationships while learning. “Minutes = sixty times hours” is easier to inspect than an unlabelled multiplication. When you become fluent, the labels can become shorter, but their meaning should remain available. A correct calculator entry is useful only if it represents the intended quantity.
A quarter of an hour is fifteen minutes. Half an hour is thirty minutes. Three quarters of an hour is forty-five minutes. These give the decimal-hour landmarks 0.25, 0.50 and 0.75. They help you recognise whether an answer is near a sensible point without calculating every digit again.
For example, 0.6 hour should be more than thirty minutes but less than forty-five minutes. Multiplying by sixty gives thirty-six minutes, which fits. A claim that 0.6 hour is six minutes fails that comparison immediately. The decimal part represents a fraction of the whole hour, not a count of minutes copied from the digit.
Landmarks are checks, not a complete conversion table. A duration such as 0.37 hour is still handled by multiplying by sixty, giving 22.2 minutes. Reference fractions tell you that it lies between fifteen and thirty minutes. They support the method while leaving the general relationship intact.
A fictional activity lasts 2.45 hours. Express its duration in hours and minutes. First separate two whole hours from 0.45 hour. Convert the fractional part: 0.45 × 60 = 27 minutes. The duration is therefore two hours twenty-seven minutes. Check by total minutes: 2.45 × 60 = 147.
Two hours twenty-seven minutes is 120 + 27 = 147 minutes, so both routes agree. A tempting answer of two hours forty-five minutes would represent 165 minutes. Its extra eighteen minutes come from interpreting forty-five hundredths of an hour as forty-five minutes rather than converting the fraction.
Notice that the original number’s final digits are not a clock reading. They belong to a decimal quantity measured in hours. If the same activity were recorded as 2:45 in an explicitly hours-and-minutes field, the duration would be different. State the format before calculating, especially when copying from a digital interface.
The table describes durations, not times of day. Its colon notation is explicitly defined as hours:minutes. Without that definition, a colon display could represent minutes:seconds instead. Each row preserves one duration across all three columns, so you can use it to check which conversion a problem requires.
| Decimal hours | Hours:minutes duration | Total minutes |
|---|---|---|
| 0.25 h | 0:15 | 15 |
| 1.50 h | 1:30 | 90 |
| 2.35 h | 2:21 | 141 |
| 2.75 h | 2:45 | 165 |
| 3.20 h | 3:12 | 192 |
The rows 2.35 and 2.75 are especially useful. Their decimal parts become twenty-one and forty-five minutes respectively. You cannot obtain the minute field just by removing the decimal point. The conversion depends on sixty minutes per hour, and the table simply records the results of that relationship.
Twenty minutes is 20/60 = 1/3 hour. The decimal expansion repeats: 0.3333… hour. The fraction is exact and often easier to use in a calculation. If a speed is twelve kilometres per hour for twenty minutes, distance is 12 × 1/3 = 4 kilometres exactly in the stated model.
Using 0.33 hour instead gives 3.96 kilometres, which is a rounded approximation introduced before the final step. The difference may matter if the task requests an exact value or a particular accuracy. Keep the fraction or enough internal precision until the answer is rounded as requested.
There is no requirement that a correctly converted time have a short terminating decimal. Some minute amounts do, such as fifteen minutes giving 0.25 hour. Others repeat. A long decimal does not mean the conversion failed. It may mean the fraction of an hour cannot be written exactly with finitely many decimal digits.
The same idea applies one unit lower. A duration of 3.5 minutes is three minutes plus half a minute, so it is three minutes thirty seconds. Multiplying the whole duration by sixty gives 210 seconds. The five after the decimal point still means five tenths of the named unit.
A duration of 3.50 minutes is not three minutes fifty seconds. Fifty seconds is 50/60 = 5/6 minute, so three minutes fifty seconds is 23/6 minutes, approximately 3.8333 minutes. The notation issue is identical to the hour-minute issue, but the smaller unit is now seconds.
When a race timer shows 3:50, check whether it means minutes:seconds. Reading it as hours:minutes would change the duration dramatically. The colon only separates fields; the interface or question defines what those fields represent. A label such as min:s removes the ambiguity and should be retained when reporting the result.
Convert 1.235 hours into hours, minutes and seconds. The whole hour is one. The remaining 0.235 hour gives 0.235 × 60 = 14.1 minutes. Keep fourteen whole minutes and convert the remaining 0.1 minute: 0.1 × 60 = 6 seconds. The duration is one hour fourteen minutes six seconds.
An alternative route gives total seconds: 1.235 × 3600 = 4446 seconds. One hour uses 3600 seconds, leaving 846. Fourteen minutes use 840, leaving six. The two methods agree and show how the units fit together. Do not read 14.1 minutes as fourteen minutes one second.
If the question asks only for the nearest minute, round the converted duration according to that instruction. Here fourteen minutes six seconds rounds to fourteen minutes within the fractional hour. Avoid rounding 1.235 hours to 1.24 hours first, since that changes the input before the requested conversion has been completed.
Suppose one task lasts one hour thirty-five minutes and another lasts 0.75 hour. Convert the first to ninety-five minutes and the second to forty-five minutes. Their total is 140 minutes, or two hours twenty minutes. The intermediate numbers share one unit, so addition preserves their meaning.
Adding 1.35 and 0.75 would give 2.10, but the first number would have been misrepresented before the addition. One hour thirty-five minutes is not 1.35 hours. Even when a final numeral looks like a plausible time, inspect the representations that entered the calculation.
You could instead work in hours: 1 + 35/60 + 3/4 = 7/3 hours. The fractional part, one third of an hour, is twenty minutes. Both methods are valid. Total minutes usually keep this particular addition simpler, while exact fractions show why the answer remains consistent across representations.
You do not always have to convert everything to one unit first. Add one hour forty-eight minutes and two hours twenty-seven minutes by combining the hour parts and minute parts: three hours seventy-five minutes. Regroup sixty of those minutes as one hour, giving four hours fifteen minutes.
The regrouping base is sixty at the hour-minute boundary. It is not one hundred, even though ordinary written arithmetic often uses decimal place value. Keeping a visible unit label beside each column helps prevent a student from treating seventy-five minutes as seventy-five hundredths of an hour.
To check, convert the inputs to 108 and 147 minutes. Their sum is 255 minutes, which is four hours fifteen minutes. The check confirms both the addition and the regrouping. Use mixed-unit arithmetic when it is comfortable, but return to total minutes if the columns become difficult to control.
Find the difference between three hours ten minutes and one hour forty-five minutes. Since ten minutes is smaller than forty-five, exchange one of the three hours for sixty minutes. The first duration becomes two hours seventy minutes. Subtracting gives one hour twenty-five minutes.
In total minutes, the same calculation is 190 − 105 = 85. Eighty-five minutes is one hour twenty-five minutes. Ordinary decimal subtraction of 3.10 − 1.45 would not represent these durations correctly because its place-value structure assumes hundredths rather than a separate minute field.
Choose one method and keep its units consistent. If you begin in hours and minutes, show the exchange. If you begin in minutes, convert both complete durations. Switching halfway between a mixed-unit record and a decimal number without a conversion is the move that creates many avoidable mistakes.
A clock reading such as 14:30 identifies a time within a day in a stated clock system. A duration of 1:30 in an hours:minutes field describes ninety minutes. They use similar punctuation but play different roles. One locates an event; the other measures how long an interval lasts.
If a lesson starts at 14:30 and lasts 1.5 hours, the ending time is 16:00 on the same day. First interpret the duration as one hour thirty minutes. Then add it to the starting time. Treating 1.5 as one hour fifty minutes would incorrectly produce 16:20.
For a journey crossing midnight, dates may be needed as well as clock readings. The series directory in the Mathematics learning hub includes the overnight-timetable guide. Use that route when the calendar boundary is the difficulty. The present lesson focuses on duration conversion; the date question is a separate part of interpreting the whole journey.
A cyclist travels at a constant twelve kilometres per hour for one hour forty minutes in an invented model. Convert the duration to hours: 1 + 40/60 = 5/3 hours. Distance equals speed multiplied by time, so the distance is 12 × 5/3 = 20 kilometres.
If a student enters 12 × 1.40, the result is 16.8 kilometres. That calculation uses a duration of one hour twenty-four minutes, because 0.40 hour is twenty-four minutes. The formula itself may be correct while the time value supplied to it is wrong. Diagnose the conversion before reteaching multiplication.
The unit check explains the needed conversion. Kilometres per hour multiplied by hours gives kilometres. If time is kept in minutes, rewrite the speed as 12/60 = 0.2 kilometre per minute. Then 0.2 × 100 = 20 kilometres. Either compatible route works; the units must agree throughout.
Question A: convert 1.8 hours into hours and minutes. Question B: convert two hours forty-two minutes into decimal hours. Question C: convert 0.35 hour into minutes. Question D: a duration display explicitly labelled min:s shows 4:18. Express the duration in decimal minutes.
Question E: add one hour twenty-five minutes and 0.6 hour, reporting the total in hours and minutes. Question F: an invented machine processes eighteen units per hour at a constant rate. How many units does the model predict in fifty minutes? Keep an exact fraction for the converted duration.
Question G: explain whether 2.40 h and 2:40 in an hours:minutes field represent the same duration. Give their total minutes and their difference. Before looking at the answers, write the unit relationship that each conversion uses. A correct final number should be supported by a clear interpretation of its source format.
For A, 0.8 × 60 = 48, so the duration is one hour forty-eight minutes. For B, forty-two minutes is 42/60 = 0.7 hour, giving 2.7 hours. For C, 0.35 × 60 = 21 minutes. For D, eighteen seconds is 18/60 = 0.3 minute, so the duration is 4.3 minutes.
For E, one hour twenty-five minutes is eighty-five minutes, and 0.6 hour is thirty-six minutes. Their total is 121 minutes, or two hours one minute. For F, fifty minutes is 5/6 hour, so the predicted quantity is 18 × 5/6 = 15 units. The constant-rate assumption belongs to the example.
For G, 2.40 h is 144 minutes, while 2:40 is 160 minutes in the stated field format. The difference is sixteen minutes. The decimal value has forty hundredths of an hour; the field record has forty minutes. Similar digits do not establish equal durations when the representations assign them different meanings.
Suppose a model journey covers twenty-seven kilometres at a constant eighteen kilometres per hour. Time is distance divided by speed: 27/18 = 1.5 hours. Converting the fractional half hour gives thirty minutes, so the journey lasts one hour thirty minutes. The decimal answer is already a valid duration in hours.
A student may correctly divide twenty-seven by eighteen, then report one hour fifty minutes. That error happens after the formula calculation, at the interpretation stage. Ask what the five in 1.5 represents. Returning to five tenths of an hour repairs the meaning without changing the correct division.
Check forward using the converted duration. Ninety minutes is 1.5 hours, and eighteen kilometres per hour for that duration gives twenty-seven kilometres. If the reported duration were one hour fifty minutes, the model distance would be thirty-three kilometres. The reverse calculation makes the mismatch visible.
A fictional room-hire exercise charges $24 per hour, proportionally for any fraction of an hour. For a booking lasting one hour forty-five minutes, time is 1.75 hours. The model cost is 24 × 1.75 = $42. The proportional charging condition is necessary for that multiplication.
If a different exercise charges for every started hour, the same duration would be charged as two whole hours, giving $48. The time conversion is unchanged; the billing rule is different. Do not use the existence of a rate to assume that every provider charges exact fractions of an hour.
These are arithmetic models, not current prices or terms for an actual business. Read the charging unit and rounding policy before interpreting a real bill. Mathematics can apply a stated rule accurately, but converting time alone does not tell you whether a fee is proportional, capped, rounded or charged in blocks.
A duration recorded as 1.23 hours corresponds to one hour thirteen minutes forty-eight seconds if the decimal value is treated as exact. The fractional 0.23 hour gives 13.8 minutes; the final 0.8 minute gives forty-eight seconds. The hundredths place in hours does not directly represent either minutes or seconds.
One hundredth of an hour is 0.01 × 3600 = 36 seconds. Therefore rounding a duration to the nearest hundredth of an hour uses thirty-six-second steps. It is a different reporting precision from rounding to the nearest minute or the nearest second. Similar decimal lengths do not imply similar time accuracy.
If 1.23 hours is a rounded record rather than an exact input, the underlying duration may differ from the exact conversion just calculated. State which interpretation the task supplies. The series guide on rounding boundaries explains that distinction; unit conversion preserves the information given but cannot restore digits already discarded.
Digital systems may display a duration as a decimal number of hours, a clock-style field, or a fraction of a day. A cell showing 1:30 is not enough by itself to identify the underlying storage rule. Read the field label, number format or documented input instructions before copying the value into a formula.
For a generic system that explicitly stores durations as fractions of a day, ninety minutes corresponds to 90/1440 = 1/16 day. Multiplying that day value by twenty-four converts it to 1.5 hours. This is a conditional mathematical example, not an assertion that every application uses the same internal representation.
When exchanging data, include a column heading such as decimal hours or total minutes. A bare column named time invites incompatible interpretations. Clear labels make the file easier for another person to inspect and prevent an apparently tidy data transfer from changing the quantity each number represents.
Which is longer: 1.7 hours or one hour forty-five minutes? Convert 1.7 hours to 102 minutes. The other duration is 105 minutes. Therefore one hour forty-five minutes is longer by three minutes. Comparing the digits seventy and forty-five would not answer the question because they refer to different kinds of parts.
You can also convert both to decimal hours: one hour forty-five minutes is 1.75 hours, which exceeds 1.70 hours by 0.05 hour. Multiplying 0.05 by sixty gives three minutes. This second route confirms that a small decimal-hour difference can represent several whole minutes.
For ordering a list of several mixed formats, total minutes or total seconds often provide a simple common scale. Keep enough precision during conversion, sort the compatible values, and then report the requested format. The common scale removes notation distractions while preserving the durations being compared.
The duration one hour twenty minutes is 4/3 hours, or 1.3333… hours. Multiplying the exact recurring value by sixty gives eighty minutes. A finite display such as 1.333 is only an approximation and gives 79.98 minutes if treated as exact. The repeating pattern and the shortened display are different numerical objects.
This connects with the series lesson on recurring decimals. The issue is not that fractions are always preferable to decimals; it is that an exact recurring value should not be confused with a finite truncation. Use 4/3 in a calculation when you need to preserve the exact duration efficiently.
If the question requests a decimal answer to three significant figures, then 1.33 hours may be the required reported approximation. Do not treat that instruction as permission to replace every earlier exact quantity with a rounded value. Calculate from the available exact information and round at the requested reporting stage.
Suppose a learner writes one hour twenty minutes = 1.20 hours, then multiplies by a correct hourly rate. The first equality is wrong. Repeating the rate formula will not repair the duration. Ask the learner to convert the twenty minutes as a fraction of sixty and rebuild the hour value.
If another learner correctly gets ninety minutes but writes ninety hours in the final answer, the numerical conversion and the unit reporting have separated. Ask what one unit of the final ninety counts. A short unit sentence can locate the error more precisely than restarting the whole question.
A third learner converts 0.25 hour by dividing by sixty. Ask whether a quarter hour should contain less than one minute or fifteen minutes. A familiar reference duration exposes the direction problem immediately. Corrective teaching works best when it addresses the actual mistaken relationship rather than giving every learner the same extra practice.
Begin with a whole hour and divide it into two equal parts. Each part contains thirty minutes. Then connect one half with the decimal 0.5. The statement 1.5 hours now has a visible structure: one whole hour plus one half hour. The explanation does not require a memorised list of every possible decimal.
Next ask about 1.25 hours. One quarter of sixty is fifteen, so the duration is one hour fifteen minutes. Then try 1.2 hours: two tenths of sixty is twelve, so it is one hour twelve minutes. Comparing these close-looking decimals helps show why each digit’s place matters.
Finally ask the learner to invent a decimal-hour value and convert it. If they choose a difficult repeating conversion in the reverse direction, keep an exact fraction. The goal is confidence in the unit relationship, not a performance in rapid mental arithmetic. A slower correct explanation is a useful foundation for later speed.
On one day, convert three decimal-hour values into minutes. On another, reverse three minute values into hours, keeping a fraction when needed. Later, mix the directions in one small set. This makes you choose an operation from the units instead of repeating one button sequence until it becomes automatic.
Include one deliberately similar pair, such as 1.45 h and one hour forty-five minutes. Explain their difference in a sentence. Then include a rate question so that the conversion has a purpose. The learner should see how a small representation error changes a later distance or model cost.
Record a correction specifically: “I copied the minute field into hundredths,” or “I divided when converting hours to smaller units.” That note identifies what to check next time. A vague label such as careless does not reveal whether the missing idea was place value, the sixty-unit relationship or the direction of conversion.
The unit agreement learning guide develops the broader habit of converting compatible quantities before combining them. Use it if time conversion is one example of a wider unit problem. The principle also appears in length, mass, capacity, area and rates, though their conversion factors differ.
The elapsed-time and timetable guide helps when the difficulty concerns start and finish readings. For the wider series, return to the Mathematics learning hub. Choose the next reading from the error in your working: format interpretation, conversion, rate selection or final-answer accuracy.
Families considering support can read the Secondary 1 Mathematics Clementi tutorial page for the teaching approach. The practical learning aim here is simple and testable: the student can explain what the decimal fraction counts and convert it correctly in both directions, even when the starting number changes.
Question H: convert 2.125 hours into hours, minutes and seconds. Question I: express forty-five minutes in decimal hours and ninety seconds in decimal minutes. Question J: a model journey of thirty kilometres at twenty-four kilometres per hour lasts how many hours and minutes?
Question K: compare 0.9 hour with fifty-five minutes. Question L: subtract one hour fifty minutes from three hours five minutes. Question M: a fictional proportional charge is $16 per hour. Find the model cost for two hours fifteen minutes, assuming no minimum charge or rounding to started blocks.
For H, 0.125 hour is 7.5 minutes, giving two hours seven minutes thirty seconds. For I, the answers are 0.75 hour and 1.5 minutes. For J, 30/24 = 1.25 hours, or one hour fifteen minutes. For K, 0.9 hour is fifty-four minutes, one minute shorter. For L, the difference is seventy-five minutes. For M, 16 × 2.25 = $36.
A learner may convert thirty minutes to 0.30 hour because thirty is thirty hundredths of one hundred. That statement concerns a hundred-part whole, but the relevant whole hour has sixty minutes. Thirty minutes is thirty sixtieths of the hour, which simplifies to one half. The denominator comes from the unit relationship.
The same error can hide in a correct-looking number. Sixty minutes written as 0.60 hour would represent only thirty-six minutes. If the conversion rule says sixty minutes is less than one hour, it contradicts the definition of the units. Testing the whole-unit case is a strong way to reject the proposed method.
Use this check whenever you are unsure about a conversion factor: what does the method do to exactly one whole larger unit? Hours to minutes should send one to sixty, and minutes to hours should send sixty to one. A rule failing either test cannot describe the stated unit conversion.
If you tell a friend that a session lasts one point five, the unit is missing. If you write 1:30, the field meanings may be missing. A clear message says ninety minutes, one hour thirty minutes, or 1.5 hours. Each gives enough information to interpret the intended duration without guessing a display convention.
For a timetable, include a starting time and the intended day where relevant. For a calculation, include the unit needed by the formula. For a data column, define whether values are decimal hours, total minutes or hours:minutes. Communication is part of preserving the quantity, especially when someone else must reuse it.
These small choices reduce ordinary planning mistakes. A learner who understands the conversion can also check whether another person’s notation is clear. They do not need to accuse the writer of being wrong; they can ask which format is intended and then translate it accurately. Mathematical precision can make a conversation easier.
Converting fifty minutes to five sixths of an hour is valid regardless of whether a machine runs steadily. Predicting output by multiplying that duration by an hourly rate additionally assumes the rate applies over the whole interval. If the machine pauses, warms up or changes speed, the model may require separate stages.
For example, a fictional device runs at twelve units per hour for twenty minutes and eighteen units per hour for thirty minutes. The first stage produces four units in the constant-rate model; the second produces nine. The total is thirteen units. The total duration is fifty minutes, but multiplying by one arbitrarily chosen rate would misrepresent the two stages.
This distinction helps you diagnose a partly correct solution. A student can convert every time correctly yet select an unsuitable model for the whole task. Keep the successful conversion, then repair the rate relationship. Mathematics becomes easier to improve when each step’s meaning is visible enough to inspect independently.
For an additional check, convert each stage to total minutes before doing the rate calculation. Twenty minutes is one third of an hour, and thirty minutes is one half. The separate contributions are therefore 12/3 and 18/2. Their sum is thirteen. Dividing thirteen by the total five sixths of an hour would give the overall average output rate, 15.6 units per hour. That combined rate describes the whole fifty-minute interval; it does not claim that the device ran at that rate at every moment.
These answers are quick return points for the most common format questions. After reading the relevant one, convert a fresh example in both directions. If you can recover the starting duration, you have a useful check that the unit relationship and arithmetic are working together.
Is 1.50 hours different from 1.5 hours?
As exact decimal-hour values, they are equal and both represent ninety minutes. The extra zero can communicate reporting precision in a measurement context, but it does not turn the decimal fraction into fifty minutes. A separate hours:minutes format would need to be stated explicitly to give that field interpretation.
Can I keep time as a fraction of an hour?
Yes. Twenty minutes is one third of an hour and fifty minutes is five sixths. Exact fractions are often convenient in rate calculations. Convert to hours and minutes at the end if requested. A repeating decimal is not required merely because the formula uses hours.
Does a colon always mean hours and minutes?
No. A timer may use minutes and seconds, while another display uses hours and minutes. Read the field label or instructions. The punctuation separates components; the surrounding definition tells you which units those components represent. Preserve that definition when you report or transfer the value.
What should I remember first?
Remember that one hour contains sixty minutes. A decimal part of an hour must be multiplied by sixty to become minutes. A minute amount must be divided by sixty to become hours. Then check that the numerical size changes in the direction expected when moving between larger and smaller units.