Average questions: working with averages instead of totals
When someone joins a group and the average changes, your child subtracts the two averages and gives a tiny answer. The question needs totals: average × number of people, before and after. This habit shows up in short questions and in the last parts of structured ones.
What it looks like
The average mass of 4 boys is 42 kg. When Ravi joins them, the average mass of the 5 boys becomes 44 kg. Find Ravi's mass.
- Total mass of 4 boys: 4 × 42 = 168 kg
- Total mass of 5 boys: 5 × 44 = 220 kg
- Ravi's mass: 220 − 168 = 52 kg
Why 2 kg is wrong: The two averages were subtracted (44 − 42 = 2) instead of the two totals.
Why children do it
- Averages are the numbers printed in the question, so children work with them directly.
- Subtracting the two averages looks like a reasonable "find the difference" step.
- Children rarely check whether a 2 kg person makes sense.
How to fix it at home
- Teach one sentence: "When the group changes, find the totals." Then total = average × how many.
- Ask your child to write a total line for the group before the change and after it.
- Check the answer against real life: no P6 boy weighs 2 kg.
- Practise both directions: a person joining, a person leaving, and a score being changed.
Questions parents ask
Why does Ravi weigh more than the old average?
Because the average went up when he joined. Anyone heavier than the average pulls it up; anyone lighter pulls it down. This is a quick check on the answer.
Where else does this habit show up?
Any question with an average and a change, such as test scores, savings or ages. Knowbot, the tutor, can walk your child through these in English, Chinese, Malay or Tamil.
Where it shows up
Other careless mistakes
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