Primary 5 Mathematics Decimals Quiz

Answer Key - Primary 5 Mathematics Quiz - Decimals

Total Marks: 50

Section A: Multiple-Choice Questions (10 marks)

1. Answer: B) 7 hundredths (2 marks)

• Explanation: In the number 3.072, the digits after the decimal point represent tenths, hundredths, and thousandths. The digit 7 is in the second place after the decimal, which is the hundredths place. So its value is 7 hundredths, or 0.07.
• Common mistake: Students may confuse hundredths with thousandths. The thousandths place is the third digit after the decimal.

2. Answer: A) 8/10 (2 marks)

• Explanation: The decimal 0.8 is read as "eight tenths." This means 8 out of 10 equal parts, which is written as the fraction 8/10.
• Common mistake: Students may think 0.8 is 8/100 because they see two digits after the decimal in other numbers. Remind them that 0.8 has only one digit after the decimal, so it is tenths.

3. Answer: D) 450 (2 marks)

• Explanation: Multiplying by 100 moves the decimal point two places to the right. 4.5 × 100 = 450.
• Working: 4.5 × 100 = 450.0 = 450.
• Common mistake: Students may move the decimal point the wrong way or only one place.

4. Answer: D) 0.101 (2 marks)

• Explanation: Compare the decimals by looking at the digits from left to right. All numbers have 0 in the ones place. In the tenths place, 0.1 and 0.101 have a 1, while 0.09 and 0.099 have a 0. So 0.1 and 0.101 are larger. Comparing 0.1 and 0.101: 0.1 is the same as 0.100, and 0.101 is larger than 0.100. So 0.101 is the largest.
• Common mistake: Students may think 0.099 is larger than 0.1 because 99 is larger than 1, forgetting to compare place values.

5. Answer: A) 0.0072 (2 marks)

• Explanation: Dividing by 1000 moves the decimal point three places to the left. 7.2 ÷ 1000 = 0.0072.
• Working: 7.2 ÷ 1000 = 0.0072.
• Common mistake: Students may move the decimal point the wrong way or only two places.

Section B: Short-Answer Questions (20 marks)

6. Answer: 0.37 (2 marks)

• Explanation: The grid has 100 squares. 37 squares are shaded. The fraction shaded is 37/100, which is written as the decimal 0.37.
• Marking: 1 mark for correct fraction 37/100, 1 mark for correct decimal 0.37.

7. Answer: 5.7 (2 marks)

• Explanation: To round to 1 decimal place, look at the digit in the second decimal place (hundredths). The number is 5.678. The hundredths digit is 7, which is 5 or more, so we round up the tenths digit. The tenths digit is 6, so it becomes 7. The answer is 5.7.
• Marking: 1 mark for identifying the correct digit to round, 1 mark for correct answer.

8. Answer: 0.045, 0.405, 0.45, 0.5 (2 marks)

• Explanation: Write all numbers with the same number of decimal places for easy comparison: 0.450, 0.405, 0.500, 0.045. Now compare: 0.045 is smallest, then 0.405, then 0.450, then 0.500.
• Marking: 1 mark for correct order, 1 mark for all correct. Deduct 1 mark for one error.

9. Answer: 0.35 (2 marks)

• Explanation: To convert a fraction to a decimal, divide the numerator by the denominator. 7 ÷ 20 = 0.35.
• Working: 20 × 0.35 = 7.00, so 7/20 = 0.35.
• Marking: 1 mark for correct division, 1 mark for correct answer.

10. Answer: 7.03 (2 marks)

• Explanation: Add the numbers, lining up the decimal points.
• Working:

    2.35
  + 4.68
  ------
    7.03
  

• Marking: 1 mark for correct method, 1 mark for correct answer.

11. Answer: 2.05 (2 marks)

• Explanation: Subtract, lining up the decimal points.
• Working:

    3.04
  - 0.99
  ------
    2.05
  

• Marking: 1 mark for correct method, 1 mark for correct answer.

12. Answer: 0.24 (2 marks)

• Explanation: Multiply the numbers as if they were whole numbers: 6 × 4 = 24. Count the total number of decimal places in the factors: 0.6 has 1 decimal place, 0.4 has 1 decimal place, total 2 decimal places. Place the decimal point in the product so that it has 2 decimal places: 0.24.
• Marking: 1 mark for correct method, 1 mark for correct answer.

13. Answer: 12 (2 marks)

• Explanation: To divide by a decimal, multiply both the dividend and divisor by 10 to make the divisor a whole number. 8.4 ÷ 0.7 becomes 84 ÷ 7 = 12.
• Marking: 1 mark for correct method, 1 mark for correct answer.

14. Answer: 0.5 m (2 marks)

• Explanation: Divide the total length by the number of pieces. 2.5 ÷ 5 = 0.5. Each piece is 0.5 m long.
• Working: 2.5 ÷ 5 = 0.5
• Marking: 1 mark for correct method, 1 mark for correct answer with unit.

15. Answer: 1500 ml (2 marks)

• Explanation: 1 litre = 1000 millilitres. So 1.5 litres = 1.5 × 1000 = 1500 ml.
• Marking: 1 mark for correct conversion factor, 1 mark for correct answer with unit.

Section C: Problem-Solving Questions (20 marks)

16. Answer: 1.0 kg (4 marks)

• Explanation:
  1. Find the total amount of flour used: 0.75 + 0.5 = 1.25 kg.
  2. Subtract the amount used from the original amount: 2.25 - 1.25 = 1.0 kg.
• Working:
  • Total flour used = 0.75 + 0.5 = 1.25 kg
  • Flour left = 2.25 - 1.25 = 1.0 kg
• Marking:
  • 1 mark for correct method to find total used
  • 1 mark for correct calculation of total used
  • 1 mark for correct method to find remaining
  • 1 mark for correct final answer with unit
• Common mistake: Students may forget to add the two amounts of flour used before subtracting.

17. Answer: 1.6 m (4 marks)

• Explanation:
  1. Find the total length of the first two pieces: 0.8 + 1.2 = 2.0 m.
  2. Subtract the total of the first two pieces from the original length: 3.6 - 2.0 = 1.6 m.
• Working:
  • Length of first two pieces = 0.8 + 1.2 = 2.0 m
  • Length of third piece = 3.6 - 2.0 = 1.6 m
• Marking:
  • 1 mark for correct method to find sum of first two pieces
  • 1 mark for correct calculation of sum
  • 1 mark for correct method to find third piece
  • 1 mark for correct final answer with unit
• Common mistake: Students may subtract only one piece from the total.

18. Answer: 5 buckets (4 marks)

• Explanation: Divide the total amount of water in the tank by the capacity of one bucket. 12.5 ÷ 2.5 = 5. So 5 buckets are needed.
• Working: 12.5 ÷ 2.5 = 125 ÷ 25 = 5
• Marking:
  • 1 mark for correct method (division)
  • 1 mark for correct conversion to whole numbers (multiply by 10)
  • 1 mark for correct calculation
  • 1 mark for correct final answer with unit
• Common mistake: Students may multiply instead of divide.

19. Answer: $2.90 (4 marks)

• Explanation:
  1. Find the cost of the pen: $0.85 +$1.20 = $2.05.
  2. Find the total cost: $0.85 +$2.05 = $2.90.
• Working:
  • Cost of pen = $0.85 +$1.20 = $2.05
  • Total cost = $0.85 +$2.05 = $2.90
• Marking:
  • 1 mark for correct method to find cost of pen
  • 1 mark for correct calculation of pen cost
  • 1 mark for correct method to find total cost
  • 1 mark for correct final answer with unit ($)
• Common mistake: Students may forget to add the cost of the pencil to the total.

20. Answer: 12 loaves (4 marks)

• Explanation: Divide the total amount of flour by the amount needed for one loaf. 5.4 ÷ 0.45.
• Working:
  • 5.4 ÷ 0.45 = 540 ÷ 45 = 12
  • The baker can make 12 loaves.
• Marking:
  • 1 mark for correct method (division)
  • 1 mark for correct conversion to whole numbers (multiply by 100)
  • 1 mark for correct calculation
  • 1 mark for correct final answer with unit
• Common mistake: Students may get a decimal answer like 12.0 and not realize it is a whole number. They may also incorrectly place the decimal point during division.