Boys and girls are in the ratio 2:5. Three boys join the group and half the girls leave. Standard PSLE ratio setup — and there is a trap in it that costs marks for no good reason.
Half of 5 units is 2.5 units. That is correct, and it is also the beginning of a long, error-prone question, because every line after it carries a decimal.
The fix takes one second. Before you halve anything, multiply the whole ratio by 2: 2:5 becomes 4:10. Now half the girls is 5 units, not 2.5, and the entire question runs on whole numbers.
Kydon shows both routes in this clip and is explicit that the decimal is not wrong — it is just harder than it needs to be. In an exam, “harder than it needs to be” is what careless mistakes are made of.
Full 9-minute lesson: https://youtu.be/JlNor59hMHo
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