Percentage after a change: dividing by the old total
Your child finds the right part but divides it by the total from before the change, so the percentage is wrong. It often shows up in the last part of a structured question, after your child has done the hard work correctly. The method marks may stay but the answer mark goes.
What it looks like
There were 200 people at a concert and 30% of them were children. Later, 50 more adults arrived. What percentage of the people at the concert were children then?
- Children: 30% × 200 = 60
- New total: 200 + 50 = 250
- Percentage of children: 60/250 × 100% = 24%
Why 30% is wrong: The 60 children were divided by the old total of 200 instead of the new total of 250.
Why children do it
- The first total is the number your child has already written down and used, so it gets reused.
- Questions often keep one group the same while another changes, and children assume the total stayed the same too.
- Percentage questions in earlier years rarely change the total halfway through.
How to fix it at home
- Ask your child to write "new total = ..." on its own line before any percentage after a change.
- Use a before-and-after table with a row for each group and a row for the total.
- Have your child ask: "percentage of which total, before or after?"
- Check the answer makes sense: if only adults arrived, the share of children must go down.
Questions parents ask
My child understands percentage. Why does this keep happening?
Understanding percentage and tracking a changing total are two different skills. This mistake is about keeping track, and it improves with questions that deliberately change the total.
Is a before-and-after table allowed in the PSLE?
Yes. Any clear working is accepted, and a table makes your child's method easy for the marker to follow.
Where it shows up
Other careless mistakes
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