Primary 4 Mathematics · Chapter 10 · Answers and explanations

Chapter 10 Revision Paper: Four Operations of Decimals (answers)

Section A · Multiple choice 10 marks

1. 3.45 + 2.8 = ?1 mark

(A) 3.73(B) 5.25(C) 6.53(D) 6.25

Answer: (D) 6.25Line up the decimal points: write 2.8 as 2.80. Hundredths: 5 + 0 = 5. Tenths: 4 + 8 = 12 tenths, write 2 and regroup 1 one. Ones: 3 + 2 + 1 = 6. The answer is 6.25.

2. 0.678 + 1.9 = ?1 mark

(A) 2.578(B) 1.578(C) 0.697(D) 2.687

Answer: (A) 2.578Write 1.9 as 1.900 so the decimal points line up. Thousandths: 8. Hundredths: 7 + 0 = 7. Tenths: 6 + 9 = 15 tenths, write 5 and regroup 1 one. Ones: 0 + 1 + 1 = 2. The answer is 2.578.

3. 7.2 − 3.46 = ?1 mark

(A) 4.46(B) 3.74(C) 3.86(D) 4.26

Answer: (B) 3.74Write 7.2 as 7.20. Hundredths: 0 − 6 is not possible, so regroup 1 tenth as 10 hundredths: 10 − 6 = 4. Tenths: 1 − 4 is not possible, so regroup 1 one: 11 − 4 = 7. Ones: 6 − 3 = 3. The answer is 3.74.

4. 5 − 2.37 = ?1 mark

(A) 3.63(B) 2.73(C) 2.63(D) 3.37

Answer: (C) 2.63Write 5 as 5.00. Regroup across the zeros: 5.00 is 4 ones, 9 tenths and 10 hundredths. Hundredths: 10 − 7 = 3. Tenths: 9 − 3 = 6. Ones: 4 − 2 = 2. The answer is 2.63.

5. A red rope is 8.4 m long. A blue rope is 2.75 m shorter than the red rope. How long is the blue rope?1 mark

(A) 11.15 m(B) 6.35 m(C) 5.75 m(D) 5.65 m

Answer: (D) 5.65 mThe blue rope is shorter, so subtract: 8.40 − 2.75 = 5.65. The blue rope is 5.65 m long.

6. 0.6 × 7 = ?1 mark

(A) 4.2(B) 42(C) 0.42(D) 4.7

Answer: (A) 4.26 tenths × 7 = 42 tenths. 42 tenths = 4 ones and 2 tenths = 4.2.

7. 3.06 × 4 = ?1 mark

(A) 12.024(B) 12.24(C) 12.4(D) 1.224

Answer: (B) 12.24Multiply from the right. Hundredths: 6 × 4 = 24, write 4 and carry 2 tenths. Tenths: 0 × 4 = 0, plus 2 = 2. Ones: 3 × 4 = 12. The answer is 12.24.

8. 0.37 × 100 = ?1 mark

(A) 3.7(B) 370(C) 37(D) 3700

Answer: (C) 37Multiplying by 100 makes the number 100 times as great. Each digit moves 2 places to the left: 0.37 becomes 37.

9. 7.35 ÷ 3 = ?1 mark

(A) 2.5(B) 2.05(C) 24.5(D) 2.45

Answer: (D) 2.457 ones ÷ 3 = 2 ones remainder 1. 13 tenths ÷ 3 = 4 tenths remainder 1. 15 hundredths ÷ 3 = 5 hundredths. So the answer is 2.45.

10. $26.40 is shared equally among 8 children. How much does each child get?1 mark

(A) $3.30(B) $3.03(C) $0.33(D) $33.00

Answer: (A) $3.3026.40 ÷ 8: 26 ones ÷ 8 = 3 ones remainder 2. 24 tenths ÷ 8 = 3 tenths. 0 hundredths ÷ 8 = 0. So each child gets $3.30.

Section B · Short answers 4 marks

11. 12.4 + 5.76 = ______1 mark

Answer: 18.16Write 12.4 as 12.40. Hundredths: 0 + 6 = 6. Tenths: 4 + 7 = 11, write 1 and regroup 1 one. Ones: 2 + 5 + 1 = 8. Tens: 1. The answer is 18.16.

12. 10 − 6.085 = ______1 mark

Answer: 3.915Write 10 as 10.000 and regroup across the zeros: 10.000 is 9 ones, 9 tenths, 9 hundredths and 10 thousandths. Thousandths: 10 − 5 = 5. Hundredths: 9 − 8 = 1. Tenths: 9 − 0 = 9. Ones: 9 − 6 = 3. The answer is 3.915.

13. 12.75 × 6 = ______1 mark

Answer: 76.5Hundredths: 5 × 6 = 30, write 0 and carry 3. Tenths: 7 × 6 = 42, plus 3 = 45, write 5 and carry 4. Ones: 2 × 6 = 12, plus 4 = 16, write 6 and carry 1. Tens: 1 × 6 = 6, plus 1 = 7. So 76.50 = 76.5.

14. 19.32 ÷ 6 = ______1 mark

Answer: 3.2219 ones ÷ 6 = 3 ones remainder 1. 13 tenths ÷ 6 = 2 tenths remainder 1. 12 hundredths ÷ 6 = 2 hundredths. The answer is 3.22.

Section C · Show your working 16 marks

15. Lina buys a bag for $12.80 and a hat for $7.45. She pays with a $50 note. How much change does she get?2 marks

Answer: $29.75Total cost: 12.80 + 7.45 = 20.25. Change: 50 − 20.25 = 29.75. She gets $29.75 change.

16. Rope A is 6.4 m long. Rope B is 1.85 m shorter than Rope A. What is the total length of the two ropes?2 marks

Answer: 10.95 mRope B: 6.40 − 1.85 = 4.55 m. Total: 6.40 + 4.55 = 10.95. The two ropes are 10.95 m long altogether.

17. Each bottle holds 1.35 ℓ of juice. How much juice is there in 6 such bottles?2 marks

Answer: 8.1 ℓ6 equal bottles, so multiply: 1.35 × 6. Hundredths: 5 × 6 = 30, write 0, carry 3. Tenths: 3 × 6 = 18, plus 3 = 21, write 1, carry 2. Ones: 1 × 6 = 6, plus 2 = 8. So 8.10 = 8.1. There is 8.1 ℓ of juice.

18. A 14.4 m ribbon is cut into 8 equal pieces. Mrs Ong uses 3 of the pieces. What is the total length of the pieces she uses?2 marks

Answer: 5.4 mOne piece: 14.4 ÷ 8 = 1.8 m (14 ones ÷ 8 = 1 one remainder 6; 64 tenths ÷ 8 = 8 tenths). Three pieces: 1.8 × 3 = 5.4 m. She uses 5.4 m.

19. Fish costs $9.65 per kg. Mr Lim buys 4 kg of fish and pays with a $50 note. How much change does he get?2 marks

Answer: $11.40Cost of fish: 9.65 × 4 = 38.60. Change: 50 − 38.60 = 11.40. He gets $11.40 change.

20. A notebook costs $3.50. The notebook and 5 identical pens cost $11.25 altogether. What is the cost of one pen?2 marks

Answer: $1.55Cost of the 5 pens: 11.25 − 3.50 = 7.75. Cost of 1 pen: 7.75 ÷ 5 = 1.55 (7 ones ÷ 5 = 1 one remainder 2; 27 tenths ÷ 5 = 5 tenths remainder 2; 25 hundredths ÷ 5 = 5 hundredths). One pen costs $1.55.

21. Bag A is 3 times as heavy as Bag B. Bag A is 7.2 kg heavier than Bag B. What is the total mass of the two bags?2 marks

Answer: 14.4 kgBag B is 1 unit and Bag A is 3 units. The difference is 3 − 1 = 2 units, and 2 units = 7.2 kg. 1 unit = 7.2 ÷ 2 = 3.6 kg. Together there are 1 + 3 = 4 units: 4 × 3.6 = 14.4. The total mass is 14.4 kg.

22. Dev has 4 times as much money as Eli. Dev gives Eli $9.60. Then they have the same amount of money. How much money did Dev have at first?2 marks

Answer: $25.60At first Dev has 4 units and Eli has 1 unit, so Dev has 3 units more than Eli (3 units). When Dev gives Eli $9.60, Dev loses $9.60 and Eli gains $9.60, so the difference changes by 2 × $9.60 = $19.20. They are now equal, so 3 units = $19.20. 1 unit = 19.20 ÷ 3 = $6.40. Dev: 4 × 6.40 = $25.60. Check: Dev has 25.60 − 9.60 = $16 and Eli has 6.40 + 9.60 = $16.

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