Primary 1 addition and subtraction in Singapore includes calculations within 100, with written algorithms as well as selected mental strategies. The most important thing for a P1 child is not to write digits quickly. It is to know why the ones belong underneath the ones, why ten ones can be exchanged for one ten, and why a subtraction answer must be checked against the original amount.
If a child writes 27 + 15 as 312, the problem is not solved by asking for more speed. The child may have added 7 + 5 to get 12, written all three digits without exchanging ten ones for a new ten, and lost the meaning of place value. Use bundles of ten, a tens–ones table and one carefully explained example before returning to a calculation.
- Official P1 content: addition and subtraction concepts, symbols +, −, =, inverse relationships, adding several one-digit numbers, calculations within 100 and algorithms.
- Key distinction: the P1 mental-calculation list names two-digit numbers plus/minus ones without renaming; written algorithms are separately included and should be taught according to the school sequence.
- Worked methods: vertical columns, aligned tens and ones, composing and decomposing tens, subtraction with and without exchange.
- Immediate check: undo an addition with subtraction, or undo a subtraction with addition.
- Policy: no national Primary 1 examination or MOE weighted assessment; these examples teach the syllabus rather than predict an exam paper.
What MOE actually lists for Primary 1 addition and subtraction
On page 31 of the MOE Primary Mathematics syllabus, updated October 2025, P1 includes the concepts of addition and subtraction; use of +, − and =; the relation between addition and subtraction; adding more than two single-digit numbers; adding and subtracting within 100; adding and subtracting using algorithms; and mental calculation within 20, of a two-digit number and ones without renaming, and of a two-digit number and tens.
This list matters because adults sometimes interpret ‘P1 within 100’ as a demand for fast mental arithmetic with any two-digit pair. That is inaccurate. The syllabus distinguishes written algorithms and selected mental-calculation situations. A child can use objects or a written method to understand 47 + 18 without being expected to solve it instantly in their head. The child’s teacher should determine exactly when a particular written algorithm is introduced.
The same document includes base-ten place value in the P1 number strand. A written algorithm is only a compact way to use that earlier understanding. The child should still be able to explain what an exchanged ten means rather than copy small carrying digits as though they are decorative marks.
First understand the columns: tens stay with tens, ones stay with ones
Write a two-column table headed Tens and Ones. Build 34 as three bundles of ten and four loose straws, and build 12 as one bundle of ten and two loose straws. The total is four bundles and six singles, or 46. The vertical written version places 34 above 12, with the 4 directly above the 2 and the 3 directly above the 1. Adding ones gives 6; adding tens gives 4. Thus 34 + 12 = 46.
The columns do not create the arithmetic. They record the value of the objects in a compressed format. If a child aligns the 1 of 12 under the 4 of 34, they may obtain a meaningless sum. The repair begins by pointing to each digit’s value, not by demanding neater penmanship alone. Have the learner place tens and ones into their corresponding boxes before writing the sum without boxes.
Worked example 1: 23 + 15 without renaming
- Make two tens and three ones for 23.
- Make one ten and five ones for 15.
- Combine the ones: 3 + 5 = 8 ones.
- Combine the tens: 2 tens + 1 ten = 3 tens.
- Write 38 and check by subtracting 15 from 38 to recover 23.
A helpful spoken explanation is: ‘I had twenty-three; adding fifteen gives three tens and eight ones, so thirty-eight.’ The child has communicated both the operation and the place-value structure. The inverse check protects against a copied or guessed answer.
Worked example 2: 46 − 23 without renaming
Build 46 as four tens and six ones. Remove three ones, leaving three ones, then remove two tens from the four tens, leaving two tens. The result is 23. In a vertical calculation, 6 − 3 = 3 in the ones column and 4 tens − 2 tens = 2 tens. Check 23 + 23 = 46. The representation shows why the method works.
What renaming means when ones cross ten
Renaming—sometimes called regrouping or exchanging—means using a different but equivalent place-value representation. Ten ones are the same quantity as one ten. In addition, when a ones-column total reaches ten or more, exchange ten ones for a ten and leave the remainder in the ones column. In subtraction, when there are not enough ones to remove, exchange one ten for ten extra ones.
Renaming does not create or destroy counters. It changes their grouping. Use a real bundle of ten that can be opened and closed. If a child can physically exchange ten singles for one bundle, the small carrying digit in a written algorithm becomes a record of a real quantity, not an arbitrary rule.
Worked example 3: 27 + 15 with renaming
- Build 27 as two tens and seven ones, and 15 as one ten and five ones.
- Combine seven and five ones to make twelve ones.
- Exchange ten of those twelve ones for one new ten, leaving two ones.
- Combine the tens: two tens plus one ten plus the exchanged ten makes four tens.
- The result is 42. Check 42 − 15 = 27 or 42 − 27 = 15.
In the vertical written algorithm, write 2 in the ones place and carry one ten into the tens column before adding the tens. Writing 312 or 32 shows that the learner may have recorded the twelve ones incorrectly or forgotten the extra ten. Look at the physical representation to identify which step was lost.
Worked example 4: 38 + 24 with renaming
Eight ones plus four ones make twelve ones. Rename as one ten and two ones. Three tens plus two tens plus the exchanged ten make six tens. The result is 62. A second method checks: 38 + 20 = 58, then add 4 to obtain 62. Showing two consistent routes is valuable because it demonstrates that the written marks record a quantity relationship.
Worked example 5: 64 − 27 with exchange
- Build 64 as six tens and four ones.
- You need to remove seven ones, but only four loose ones are available.
- Open one ten-bundle, leaving five full tens and fourteen loose ones.
- Remove seven ones from fourteen, leaving seven ones.
- Remove two tens from five tens, leaving three tens. The result is 37.
- Check 37 + 27 = 64.
A common error is to compute 7 − 4 = 3 in the ones column and 6 − 2 = 4 in the tens column, giving 43. This does not describe 64 − 27. Ask the learner which quantity is being removed and from which amount. The exchange is necessary precisely because four ones cannot supply seven removed ones.
Worked example 6: 50 − 16 with a zero in the ones column
Start with five tens and no ones. To remove six ones, open one of the tens: four tens and ten ones remain. Subtract six ones to leave four ones; subtract one ten from the remaining four tens to leave three tens. The answer is 34. The check is 34 + 16 = 50. A student who computes 0 − 6 as though it were 6 has lost the direction of subtraction.
The written method and the mental method are related, not identical
Some calculations within 100 are straightforward to do mentally because no exchange is needed. For 52 + 4, add four ones to the existing two ones; the answer is 56. For 67 − 5, remove five from seven ones, leaving 62. For 43 + 20, add two tens to four tens, reaching 63. The official P1 mental-calculation wording specifically highlights this kind of structure.
By contrast, 37 + 8 crosses a ten boundary. A child may solve it mentally by making forty—37 + 3 + 5 = 45—or use a written or concrete method. It is not accurate to read the official mental-calculation clause as requiring every two-digit-plus-ones calculation involving renaming to be performed mentally. Treat such questions as guided work within the wider addition and subtraction strand according to the school’s teaching progression.
Checking with the inverse operation
Addition and subtraction are inverse relationships. If 28 + 16 = 44, then 44 − 16 should return 28. If 73 − 29 = 44, then 44 + 29 should return 73. These checks are not merely extra arithmetic. They ask whether the result is consistent with the original amount, which is a core mathematical reasoning habit.
Be careful when teaching a check: a learner who repeats the same incorrect written method may reproduce the same wrong answer. After an algorithm, try a different representation, such as tens and ones or a number line, or estimate whether the result is near a reasonable quantity. For example, 48 + 27 must be more than 48, so a written result of 21 is impossible even before the exact sum is found.
Forty original calculation exercises with answers
These examples are grouped by purpose. They are original eduKate practice, not official P1 examination questions. Some involve renaming as part of written-algorithm instruction; follow the school’s sequencing and allow place-value blocks where needed.
Set A — align tens and ones, no renaming
- 23 + 14 = ? Answer: 37. Ones total 7, no new ten required.
- 34 + 12 = ? Answer: 46. Ones total 6, no new ten required.
- 52 + 16 = ? Answer: 68. Ones total 8, no new ten required.
- 41 + 27 = ? Answer: 68. Ones total 8, no new ten required.
- 35 + 24 = ? Answer: 59. Ones total 9, no new ten required.
- 63 + 15 = ? Answer: 78. Ones total 8, no new ten required.
- 76 + 12 = ? Answer: 88. Ones total 8, no new ten required.
- 28 + 31 = ? Answer: 59. Ones total 9, no new ten required.
- 47 + 22 = ? Answer: 69. Ones total 9, no new ten required.
- 50 + 29 = ? Answer: 79. Ones total 9, no new ten required.
Set B — subtract without exchange
- 68 − 25 = ? Answer: 43. Subtract 5 ones from 8 ones without exchange.
- 79 − 46 = ? Answer: 33. Subtract 6 ones from 9 ones without exchange.
- 54 − 21 = ? Answer: 33. Subtract 1 ones from 4 ones without exchange.
- 86 − 34 = ? Answer: 52. Subtract 4 ones from 6 ones without exchange.
- 75 − 13 = ? Answer: 62. Subtract 3 ones from 5 ones without exchange.
- 49 − 17 = ? Answer: 32. Subtract 7 ones from 9 ones without exchange.
- 93 − 30 = ? Answer: 63. Subtract 0 ones from 3 ones without exchange.
- 62 − 12 = ? Answer: 50. Subtract 2 ones from 2 ones without exchange.
- 88 − 44 = ? Answer: 44. Subtract 4 ones from 8 ones without exchange.
- 37 − 16 = ? Answer: 21. Subtract 6 ones from 7 ones without exchange.
Set C — add with exchange, using written or concrete representation
- 27 + 15 = ? Answer: 42. Ones total 12: rename one ten and leave 2 ones.
- 38 + 24 = ? Answer: 62. Ones total 12: rename one ten and leave 2 ones.
- 46 + 17 = ? Answer: 63. Ones total 13: rename one ten and leave 3 ones.
- 58 + 13 = ? Answer: 71. Ones total 11: rename one ten and leave 1 ones.
- 29 + 14 = ? Answer: 43. Ones total 13: rename one ten and leave 3 ones.
- 36 + 27 = ? Answer: 63. Ones total 13: rename one ten and leave 3 ones.
- 47 + 28 = ? Answer: 75. Ones total 15: rename one ten and leave 5 ones.
- 55 + 16 = ? Answer: 71. Ones total 11: rename one ten and leave 1 ones.
- 68 + 23 = ? Answer: 91. Ones total 11: rename one ten and leave 1 ones.
- 19 + 27 = ? Answer: 46. Ones total 16: rename one ten and leave 6 ones.
Set D — subtract with exchange, using written or concrete representation
- 64 − 27 = ? Answer: 37. Exchange one ten: 14 − 7 = 7 ones.
- 50 − 16 = ? Answer: 34. Exchange one ten: 10 − 6 = 4 ones.
- 71 − 24 = ? Answer: 47. Exchange one ten: 11 − 4 = 7 ones.
- 83 − 45 = ? Answer: 38. Exchange one ten: 13 − 5 = 8 ones.
- 62 − 28 = ? Answer: 34. Exchange one ten: 12 − 8 = 4 ones.
- 90 − 38 = ? Answer: 52. Exchange one ten: 10 − 8 = 2 ones.
- 43 − 16 = ? Answer: 27. Exchange one ten: 13 − 6 = 7 ones.
- 82 − 37 = ? Answer: 45. Exchange one ten: 12 − 7 = 5 ones.
- 75 − 48 = ? Answer: 27. Exchange one ten: 15 − 8 = 7 ones.
- 61 − 29 = ? Answer: 32. Exchange one ten: 11 − 9 = 2 ones.
Eight recurring algorithm mistakes, with exact repairs
Misaligned columns
What you may see: A child writes the 1 from 15 beneath the 7 in 27, then combines digits in the wrong places. Repair: Use a marked tens/ones chart. Ask the learner to state each digit’s value aloud as it is positioned, then gradually remove the chart. After practising, test a new problem without giving the first step.
Writing twelve in the ones column
What you may see: Seven ones and five ones are added as 12, and both digits are written in the ones area. Repair: Exchange ten of the twelve ones for one complete ten and write the remaining two ones. The extra ten belongs in the tens column. After practising, test a new problem without giving the first step.
Forgetting the carried ten
What you may see: The child records the ones correctly but adds only the original tens after renaming. Repair: Keep the newly formed ten-bundle physically beside the original bundles. Ask the child to count it before recording the tens total. After practising, test a new problem without giving the first step.
Subtracting the smaller digit from the larger regardless of position
What you may see: For 64 − 27, the child does 7 − 4 instead of recognising that four ones cannot supply seven. Repair: Model removal from the total 64. Exchanging one ten makes fourteen ones, from which seven can be removed. After practising, test a new problem without giving the first step.
Forgetting to reduce tens after borrowing
What you may see: The student changes four ones to fourteen but leaves six tens, creating extra value. Repair: After exchanging one ten, count the complete bundles again: there are now five tens. Ask whether any amount was added during the exchange; it was not. After practising, test a new problem without giving the first step.
Treating zero as six, nine or another digit
What you may see: The learner sees 50 − 16 and tries to remove six ones from zero without regrouping. Repair: Open a tens bundle to form ten ones. Demonstrate the exchange with 50 before applying the written algorithm. After practising, test a new problem without giving the first step.
Skipping the inverse check
What you may see: A result of 21 for 48 + 27 is accepted without noticing that adding made the starting amount smaller. Repair: Ask for a rough size comparison, then use subtraction or counters to recover the original quantity. After practising, test a new problem without giving the first step.
A copying error mistaken for a concept error
What you may see: A learner can explain 28 + 15 = 43 but writes 34 in a final box. Repair: Compare the oral quantity with the written numeral. Repair tens/ones notation separately from the already-correct addition reasoning. After practising, test a new problem without giving the first step.
A 90-minute small-group tutorial structure: what is actually useful
A careful small-group lesson can begin with five to ten minutes of retrieval: place value, an addition fact and one subtraction check. The tutor then observes one written example performed independently. If the student misaligns columns, that becomes the immediate teaching point. If the student knows the layout but loses the exchanged ten, the tutor models that specific exchange with blocks and returns to the written calculation.
Guided work should include one simple no-exchange addition, one exchange example and an inverse check, with subtraction introduced at a pace that preserves understanding. The student then solves new questions without copying the teacher’s exact numbers. A short review of the first wrong inference is more useful than a whole lesson of answer-marking. At Primary 1, the aim is mathematical independence, not simulated high-stakes testing.
Four-week practice: from materials to written working
Week 1: put every number in its place
Use bundles of ten and single counters. Arrange 23 + 14 and 67 − 25 as quantities before writing them vertically. Concentrate on alignment and meaning; there is no need for a timed score. Keep the volume manageable and follow the teacher’s pace. A child who cannot explain a new problem should return to the smallest unstable step, not be assigned more of the same at speed.
Week 2: secure the no-exchange algorithm
Practise written addition and subtraction where the ones can be combined or removed without renaming. Ask the child to check one problem by the inverse operation and one by a number model. Keep the volume manageable and follow the teacher’s pace. A child who cannot explain a new problem should return to the smallest unstable step, not be assigned more of the same at speed.
Week 3: introduce exchange visibly
Build 27 + 15 and 64 − 27 with counters. Physically make or break a ten-bundle. Only then record the renamed ten in vertical working; use a different numerical example to test transfer. Keep the volume manageable and follow the teacher’s pace. A child who cannot explain a new problem should return to the smallest unstable step, not be assigned more of the same at speed.
Week 4: decide, solve and self-check
Mix sums and differences, some requiring exchange and some not. The student must select the operation from a clear sentence, align values, calculate and explain a reasonableness or inverse check. Keep the volume manageable and follow the teacher’s pace. A child who cannot explain a new problem should return to the smallest unstable step, not be assigned more of the same at speed.
What to teach before calling an error ‘careless’
Children sometimes receive the label ‘careless’ for quite different reasons. A reversed digit may be a written-number issue; a missed carried ten is a place-value issue; a minus sign overlooked in a story is a question-reading issue. To distinguish them, ask the child to read the equation aloud, build the quantities and describe what each written digit means. The first point where the explanation stops making sense identifies the next teaching step.
A child who gets a calculation right with counters but wrong in columns may be ready for more practice transferring between representations. A child who copies the teacher’s vertical working without understanding the exchanged bundle may still need concrete examples. A child who performs the arithmetic perfectly but adds when the story requires subtraction should return to the meaning of the operation. These are not interchangeable problems.
Frequently asked questions
Are there official Primary 1 Mathematics exam questions for 2027?
No. MOE removed weighted assessments and examinations for P1 and P2. SEAB runs national examinations at other levels, not a P1 national paper. These are original curriculum-based learning examples.
Does P1 have addition and subtraction within 100?
Yes. The MOE syllabus explicitly lists adding and subtracting within 100 and using algorithms.
Must P1 students mentally calculate every two-digit sum?
No. The named mental-calculation cases are within 20, two-digit plus/minus ones without renaming, and two-digit plus/minus tens. Written algorithms are a separate listed expectation.
What does renaming mean?
It means exchanging ten ones for one ten or one ten for ten ones while keeping the total quantity unchanged. Concrete counters make this relationship visible.
Why is 27 + 15 not 312?
Because twelve ones must be renamed as one ten and two ones. Together with the other tens, the total is 42.
Why does 50 − 16 require exchange?
There are zero loose ones in 50, so one complete ten must be exchanged for ten ones before removing six ones.
Is carrying the same as exchanging a ten?
The small carried digit in a vertical algorithm records the additional ten created by the exchange. The symbol has a quantity meaning.
Can a P1 student use a drawing instead of a formal column algorithm?
Drawings and place-value blocks can build understanding and support teaching. Follow school expectations about recording an algorithm, but do not sacrifice concept for unexplained symbols.
How should I check an addition answer?
Use a related subtraction or reconstruct the quantities with a different model. Also check whether the total is reasonable for the amounts added.
How should I check a subtraction answer?
Add the amount removed back to the remainder. The result should recover the original whole.
Should I insist on many timed worksheets?
Not for a year without weighted exams. A few independent, well-explained examples reveal learning more clearly than forced speed.
When does tuition help?
When a repeatable error persists after school feedback. Small-group teaching can examine the exact exchange, alignment or interpretation that needs repair.
Ten extra mistakes that become visible when a child explains aloud
Ask the child to talk through 43 + 28. If they first say ‘three plus eight makes eleven’ and then immediately write the two digits of eleven in the answer row, pause before continuing. Build the eleven ones physically and exchange ten for a tens bundle. The learner should then describe the ones that remain and the extra ten that must be included. The error was not a failure to add three and eight; it was a failure to represent eleven ones correctly in a two-column number.
In a subtraction such as 72 − 35, ask what seven tens and two ones mean before doing anything. If the student tries two minus five and cannot explain the answer, use one tens bundle to rename the quantity as six tens and twelve ones. Now subtract five ones to obtain seven and three tens from six to obtain three tens. The result is 37. A small mistake in the exchanged tens can be identified and repaired without reteaching the meaning of subtraction.
Another useful check is 61 − 29. It requires the student to rename 61 as five tens and eleven ones. Removing nine ones leaves two, and removing two tens from five leaves three tens, so the result is 32. Verify 32 + 29 = 61. A mistaken answer of 42 may indicate that the student forgot to reduce the tens after exchanging; a mistaken answer of 38 may indicate that they reversed the ones subtraction. These errors have different causes.
Compare two alternative ways of finding 46 + 17: the written algorithm renames 6 + 7 = 13 ones, while a flexible mental approach could add 10 to reach 56 and then 7 to reach 63. Both produce 63, but the writing and reasoning steps differ. When a student can make sense of both, the written algorithm becomes less mechanical. Avoid demanding both approaches for every task; use comparison strategically to deepen understanding.
Consider the apparently easy 80 − 40. A learner may write 4 because they calculate eight minus four without accounting for tens. Show eight bundles of ten and remove four bundles; four bundles remain, worth 40. This is a place-value error rather than an arithmetic fact error. Return to the bundles, ask why the answer has a zero in the ones place and test a new example such as 70 − 20.
The same principle applies to 30 + 6. If the child writes 9, they may be combining the digits 3 and 6 as ones. Build three tens and six singles, say ‘thirty-six’, and record the digits in columns. Ask what number the child would have if the three tens were replaced with three single counters; then it really would be nine. The counterfactual comparison makes the source of the mistake clear.
When students work on different representations, some errors disappear in one format and return in another. Record this rather than concluding that the child has mastered the topic after one right answer. A learner who can solve 38 + 24 with blocks should next try a drawn tens-and-ones diagram, and finally vertical written working. If the last stage fails, bridge the representations rather than adding harder numerical examples.
Parent questions to ask after a wrong answer
- Which digit names the tens and which digit names the ones?
- Can you show the total or the amount removed with real objects?
- Did any group of ten need to be made or opened?
- Where did you record that exchanged ten in the written work?
- If your answer were right, would the inverse calculation recover the starting number?
- Can you solve a similar question with smaller numbers first?
- Which step felt uncertain, and what could you check without seeing my answer?
Use these prompts selectively. A child needs enough thinking space to notice and correct the error; seven questions fired in quick succession can be more confusing than one clear cue. Start with the question that targets the most likely cause. Only after the child understands the problem should you move to a new one. Progress is not the absence of mistakes; it is increased ability to identify and repair them.
One final parent check: did the child actually exchange a ten?
After a child solves a written calculation with renaming, ask them to explain where the extra ten came from or which bundle of ten was opened. For 27 + 15, twelve ones were made from seven and five; ten of those ones became an additional ten, leaving two ones. For 64 − 27, one of the six tens was opened into ten singles so that seven ones could be removed. Both actions preserve the quantity that existed before any addition or subtraction. If the learner cannot explain that, briefly return to straws or base-ten blocks, then retry the written calculation.
Do not treat the presence of a small carried digit as proof of understanding. A child may copy the layout correctly while forgetting its meaning on the next question. A useful next step is a fresh sum with different numbers and no adult hint, followed by an inverse check. Independent transfer, not the number of completed written columns, is the meaningful result.
Next reading and official sources
For prerequisites, start with P1 Numbers to 100: Tens and Ones, then Number Bonds and Making Ten. For interpreting stories use P1 Addition and Subtraction Word Problems. The eduKate Mathematics Learning Hub connects the wider sequence.
Primary sources: MOE Primary Mathematics Syllabus, updated October 2025, P1 page 31; MOE assessment policy; SEAB PSLE information. Checked 12 November 2026 for the 2027 school year. This is independent educational content, not an examination paper issued or endorsed by MOE or SEAB.