PSLE Mathematics Learning Guide · Guide 44
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Mass problems often look easy until kilograms and grams appear together. A learner may add 2 kg and 350 g as though they were both ordinary whole numbers, or compare 950 g with 1.2 kg without first putting them on the same scale.
Choose one working unit, convert all relevant masses into that unit, perform the calculation, then convert the final answer only if another form is requested.
Guide 2 owns the general unit-alignment rule. Guide 38 covers reading a measuring scale. This page is a focused mass-application workshop: totals, differences, repeated equal masses, equal sharing, missing quantities and practical checks.
The MOE Primary Mathematics syllabus updated October 2025 provides curriculum context. All examples below are original eduKate teaching material.
Core mass relationship
1 kg = 1000 g.
Therefore 2.4 kg = 2400 g, 3750 g = 3.75 kg, and 1 kg 250 g = 1250 g.
The thousand-to-one relationship is the first control point in every mixed kg/g calculation.
Add mixed masses after conversion
2 kg 350 g + 1 kg 875 g.
Work in grams:
2350 + 1875 = 4225 g = 4 kg 225 g.
The same result can be obtained by adding the kilogram and gram parts separately, then regrouping 1225 g as 1 kg 225 g.
Subtraction may cross a kilogram boundary
5 kg 100 g − 2 kg 650 g.
Convert to grams:
5100 − 2650 = 2450 g = 2 kg 450 g.
This is often clearer than regrouping one kilogram as 1000 g mentally inside a column subtraction.
Compare only after the units agree
Which is heavier: 1.35 kg or 1275 g?
1.35 kg = 1350 g.
1350 g > 1275 g, so 1.35 kg is heavier by 75 g.
Comparing 1.35 and 1275 as bare numerals would ignore the unit scale.
Repeated equal masses use multiplication
Eight identical packets each have mass 375 g.
Total = 8 × 375 = 3000 g = 3 kg.
Keep “g per packet” attached to the repeated quantity until the total is found.
Equal sharing uses division
A 4.8 kg quantity is divided equally into 6 packets.
4.8 kg ÷ 6 = 0.8 kg = 800 g per packet.
Check: 6 × 800 g = 4800 g.
Recover a missing mass from a total
Three parcels have combined mass 7.5 kg. Two masses are 2.35 kg and 1875 g.
1875 g = 1.875 kg.
Known total = 4.225 kg.
Missing mass = 7.5 − 4.225 = 3.275 kg = 3 kg 275 g.
Net mass and container mass must stay separate
A jar and its contents have total mass 2.4 kg. The empty jar has mass 350 g.
2.4 kg = 2400 g.
Contents = 2400 − 350 = 2050 g = 2.05 kg.
This structure appears whenever a container contributes to the measured total.
Several containers can create a hidden repeated mass
Five identical empty boxes each have mass 120 g. Goods inside all five boxes have total mass 3.4 kg.
Total box mass = 5 × 120 = 600 g = 0.6 kg.
Combined packed mass = 3.4 + 0.6 = 4 kg.
Do not subtract container mass unless the question asks for contents from a packed total.
A fraction of a mass uses fraction multiplication
Find 3/5 of 2 kg.
2 kg = 2000 g.
3/5 × 2000 = 1200 g = 1.2 kg.
Use the representation that makes the arithmetic clear. Guide 18 covers fraction operations more broadly.
A percentage of a mass follows the same reference-whole rule
A 5 kg bag loses 12% of its mass.
12% of 5 kg = 0.6 kg.
Remaining mass = 4.4 kg.
The percentage applies to the original 5 kg unless the problem states a different reference mass.
Main worked workshop: sixteen original mass problems
1.
3.6 kg to grams.
Answer: 3600 g.
2.
4850 g to kilograms.
Answer: 4.85 kg.
3.
2 kg 675 g + 1 kg 850 g.
Answer: 4 kg 525 g.
4.
6 kg − 2 kg 750 g.
Answer: 3 kg 250 g.
5.
Compare 1.8 kg and 1750 g.
Answer: 1.8 kg is heavier by 50 g.
6.
Seven packets each 425 g.
Answer: 2975 g = 2.975 kg.
7.
5.4 kg divided equally into 9 packets.
Answer: 0.6 kg = 600 g each.
8.
Total 8 kg; known masses 2.25 kg, 1850 g and 3.1 kg. Find missing.
Reasoning: 2.25 + 1.85 + 3.1 = 7.2 kg; missing = 0.8 kg.
9.
A packed parcel is 3.5 kg. Empty box is 280 g. Contents?
Answer: 3220 g = 3.22 kg.
10.
Four empty jars are 150 g each. Contents total 2.8 kg. Packed total?
Answer: 600 g + 2800 g = 3400 g = 3.4 kg.
11.
Half of 3.6 kg.
Answer: 1.8 kg.
12.
25% of 2.4 kg.
Answer: 0.6 kg = 600 g.
13.
A learner adds 2 kg + 350 g and writes 352 kg.
Repair: 2 kg = 2000 g; total 2350 g = 2.35 kg.
14.
A learner says 950 g is heavier than 1.2 kg because 950 > 1.2.
Repair: 1.2 kg = 1200 g, which is heavier.
15.
A 10 kg sack has 3.25 kg and 1850 g removed. Remaining?
Answer: 10 − 3.25 − 1.85 = 4.9 kg.
16.
Six identical objects have total mass 4.2 kg. Mass of one?
Answer: 0.7 kg = 700 g.
A mass-check routine
Unit check: Have kg and g been aligned?
Direction check: If mass is removed, should the answer be smaller than the original?
Reconstruction check: Do the parts add back to the total?
Context check: Is the question asking gross mass, container mass or contents mass?
Common mass errors
Error 1: use 1 kg = 100 g instead of 1000 g.
Error 2: add kg and g as bare numerals.
Error 3: compare 1.5 and 900 without unit conversion.
Error 4: confuse container mass with contents mass.
Error 5: divide a total but forget the unit for one share.
Independent transfer check
- 4.25 kg to grams.
- 7350 g to kilograms.
- 3 kg 850 g + 2 kg 475 g.
- 7 kg 100 g − 3 kg 650 g.
- Eight packets of 275 g. Find total mass.
- 6.4 kg divided into 8 equal packets. Find one packet in grams.
- Compare 2.03 kg and 1990 g.
- A packed box is 4.2 kg and the empty box is 350 g. Find contents.
Independent-check answers
1. 4250 g.
2. 7.35 kg.
3. 6 kg 325 g.
4. 3 kg 450 g.
5. 2200 g = 2.2 kg.
6. 800 g.
7. 2.03 kg = 2030 g, so it is heavier by 40 g.
8. 4200 − 350 = 3850 g = 3.85 kg.
Parent and tutor guide
Circle the unit beside every mass before calculation. Ask the learner to choose one working unit and stay in it until the arithmetic is complete.
For container problems, draw three labels: packed total, empty container, contents. Decide whether the task is adding or removing the container contribution.
After one guided problem, change both the numbers and the final requested unit. Independent transfer means the child still aligns kg and g without being reminded.
The learner’s final card
Which masses are in kilograms? Which are in grams? What single unit will I use? Am I finding total mass, difference, equal share, container mass or contents mass? Can I rebuild the total to check?
Complete Batch 11
Use Guide 41: Equivalent Fractions, Guide 42: Compare and Order Decimals, and Guide 43: Length Problems in Mixed Units.
Return to the PSLE Learning Guide Mathematics route.
Sources and boundaries
MOE Primary Mathematics syllabus, updated October 2025; SEAB 2026 PSLE formats.
All examples and solutions are original eduKate teaching material. This focused mass workshop routes to the general units owner rather than replacing it.