The average mass of 4 bags of rice is 3.75 kg. When a fifth bag is added, the average mass of all 5 bags becomes 3.5 kg. Find the mass of the fifth bag.
Need a hint?
Work with totals, not averages: find the total of the 4 bags and the total of the 5 bags first.
Worked answer
- Total mass of the first 4 bags = 4 × 3.75 kg = 15 kg.
- Total mass of all 5 bags = 5 × 3.5 kg = 17.5 kg.
- Mass of the fifth bag = 17.5 kg − 15 kg = 2.5 kg.
Answer: 2.5 kg
The mistake this question tests
Habit
Average and total
Pupils often average the two averages, (3.75 + 3.5) ÷ 2, which ignores how many bags each average covers.
How to fix it at home →Wrong answers children often give:
- 3.625 Average and total
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See the answer →Practise PSLE whole numbers until the mistakes stop.
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