PRIMARY 4 MATHEMATICS LEARNING GUIDE · BATCH 6 · GUIDE 23
Mass problems become much easier when students keep the unit attached to the amount. Three kilograms and 250 grams is one physical mass expressed in compound units. The same mass can be written as 3,250 grams without changing how heavy the object or collection is.
This guide develops kilograms, grams, compound units, conversion, addition and subtraction, comparison, equal sharing, estimation and multi-step word problems. The central idea is to preserve the mass while changing its numerical representation.
Series route: return to the Primary 4 Mathematics Learning Hub. Primary 4 measurement includes working with mass in compound units and conversion between larger and smaller units; see the MOE Primary Mathematics Syllabus, updated October 2025.
1. Kilograms and grams describe the same kind of quantity
Kilograms and grams are units of mass. A kilogram is larger than a gram, so the numerical value becomes larger when a mass is rewritten entirely in grams.
The object does not become heavier merely because the number changes.
2. Core relationship
1 kg = 1,000 g.
This fact connects compound masses such as 2 kg 350 g to a single smaller unit.
3. Convert kilograms to grams
5 kg = 5 × 1,000 = 5,000 g.
Because grams are smaller units, more of them are needed to represent the same mass.
4. Convert compound kilograms and grams to grams
3 kg 275 g = 3,000 g + 275 g = 3,275 g.
Convert the kilogram part first, then combine like units.
5. Convert grams to kilograms and grams
4,860 g contains four complete kilograms and 860 grams.
Therefore 4,860 g = 4 kg 860 g.
6. Compare masses after unifying units
Which is heavier: 2 kg 80 g or 2,150 g?
2 kg 80 g = 2,080 g.
Therefore 2,150 g is heavier by 70 g.
7. Add masses by converting to grams
Add 1 kg 650 g and 2 kg 780 g.
1,650 g + 2,780 g = 4,430 g.
Answer: 4 kg 430 g.
8. Add masses by regrouping
Add kilograms and grams separately:
1 kg + 2 kg = 3 kg.
650 g + 780 g = 1,430 g = 1 kg 430 g.
Total = 4 kg 430 g.
This is the same mass as the conversion route.
9. Subtract compound masses
Find 5 kg 200 g − 2 kg 750 g.
Convert: 5,200 g − 2,750 g = 2,450 g.
Answer: 2 kg 450 g.
10. Regroup one kilogram when subtracting
5 kg 200 g can be rewritten as 4 kg 1,200 g.
Then 4 kg 1,200 g − 2 kg 750 g = 2 kg 450 g.
The representation changed; the starting mass did not.
11. Total mass of several objects
A parcel contains items of mass 1 kg 250 g and 850 g.
1,250 g + 850 g = 2,100 g = 2 kg 100 g.
12. Mass remaining after removal
A container holds 6 kg of material. 2 kg 375 g is removed.
6,000 g − 2,375 g = 3,625 g.
Remaining mass = 3 kg 625 g.
13. Reconstruct an original mass
After 1 kg 480 g is removed, 2 kg 320 g remains.
Original mass = 1,480 g + 2,320 g = 3,800 g = 3 kg 800 g.
The removed and remaining parts reconstruct the original whole.
14. Equal packets use multiplication
One packet has mass 450 g. Four identical packets have mass:
450 × 4 = 1,800 g = 1 kg 800 g.
The multiplier counts equal packets.
15. Equal sharing uses division
A total mass of 2,400 g is shared equally among six bags.
2,400 ÷ 6 = 400 g per bag.
The quotient is a mass because the total mass is being divided into equal shares.
16. A remainder needs interpretation
2,450 g is divided equally among six bags using whole grams.
2,450 ÷ 6 = 408 remainder 2.
If the context requires whole grams and equal bags, each bag can receive 408 g with 2 g left over. If finer measurement is permitted, the situation would need a more precise unit or decimal representation.
Do not hide the remainder when the physical situation still contains it.
17. Estimate before accepting an answer
4 kg 900 g is almost 5 kg. A conversion result of 490 g is far too small.
Similarly, adding about 2 kg and 3 kg should produce a result around 5 kg, not 50 kg.
18. Choose a sensible unit
The mass of a schoolbag may be conveniently described in kilograms and grams. The mass of a small paper clip is more naturally described in grams than kilograms.
Choosing a suitable unit improves communication and helps detect unreasonable numbers.
19. Do not combine kilograms and grams as bare numbers
3 kg + 250 g is not 253 kg.
Either write 3 kg 250 g or convert the entire mass to 3,250 g.
The unit determines what each number counts.
20. Common mass errors
| Error | Weak link | Repair question |
|---|---|---|
| 2 kg 80 g written as 2,800 g | Compound place value | How many grams are in 2 kg before adding 80 g? |
| 4,860 g written as 4 kg 86 g | Remainder misread | What remains after removing 4,000 g? |
| kg and g added without conversion | Unlike units | What common unit will make the calculation clear? |
| Mass removed but final answer grows | Direction check missing | Should the remaining mass be smaller? |
| Answer lacks a unit | Quantity incomplete | What is being measured? |
21. Practice laboratory
- Convert 7 kg to grams.
- Convert 3 kg 450 g to grams.
- Convert 5,208 g to kg and g.
- Compare 2 kg 75 g and 2,120 g.
- Add 1 kg 825 g and 2 kg 650 g.
- Subtract 2 kg 780 g from 6 kg 100 g.
- A sack has mass 8 kg. 3 kg 425 g is removed. Find the remainder.
- One packet has mass 375 g. Find the mass of six packets.
- 2,800 g is divided equally among seven bags. Find each bag’s mass.
- After removing 1 kg 650 g, 2 kg 125 g remains. Find the original mass.
- Find the difference between 4 kg 80 g and 3,950 g.
- Convert 9 kg 5 g to grams.
- Convert 10,009 g to kg and g.
- Add 850 g, 1 kg 275 g and 2 kg.
- A 5 kg package has 1,750 g removed. Find the remainder in grams.
- Estimate the total of 2 kg 950 g and 3 kg 120 g.
- Which unit is more sensible for a watermelon: kg or mg?
- Explain why 2 kg 500 g is not 2.500 g.
- A total of 2,450 g is shared into six equal whole-gram bags. State the quotient and remainder.
- Give an inverse check for 6,000 g − 2,375 g = 3,625 g.
22. Explained answers
1. 7,000 g.
2. 3,450 g.
3. 5 kg 208 g.
4. 2 kg 75 g = 2,075 g, so 2,120 g is heavier by 45 g.
5. 4 kg 475 g.
6. 3 kg 320 g.
7. 4 kg 575 g.
8. 2,250 g = 2 kg 250 g.
9. 400 g.
10. 3 kg 775 g.
11. 4,080 − 3,950 = 130 g.
12. 9,005 g.
13. 10 kg 9 g.
14. 4,125 g = 4 kg 125 g.
15. 5,000 − 1,750 = 3,250 g.
16. About 6 kg.
17. Kilograms.
18. “2 kg 500 g” is a compound mass equal to 2,500 g. “2.500 g” would mean 2.5 grams.
19. 408 g each, remainder 2 g.
20. 3,625 + 2,375 = 6,000 g.
23. Teaching routine: preserve the physical mass
Ask what the quantity measures, what unit each number uses and whether a conversion is needed. After calculation, compare the result with the starting scale and use an inverse operation where possible.
The reusable route is: name unit → convert → calculate → regroup → estimate → verify.
24. Where this guide connects next
Volume of liquid uses the same larger-unit/smaller-unit logic. A litre and one thousand millilitres represent the same amount of liquid just as one kilogram and one thousand grams represent the same mass.
Continue to Volume of Liquid: Litres, Millilitres and Compound Units →
Editorial control: Wintour House V1.0 · CivDJ · eduKate Publishing.
