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Mrs Tan and her son — money made equal
Mrs Tan had 4 times as much money as her son. After she gave him $18, the two of them had the same amount of money. How much money did Mrs Tan have at first?
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The pattern behind it: Constant total
Money moves between two people, so their total never changes.
In this one. A transfer does not change the combined total, so the same 5 units are re-split equally, and Mrs Tan's drop in units represents the $18 she gave away.
How to spot it
One person gives, sells or loses an amount to the other, and nobody outside the two people gains or loses anything.
Common mistake
Children draw different totals before and after, or forget that moving an amount from one person to the other changes the gap between them by twice that amount.
Try one like it
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All 33 patterns, one to a page — the 19 behind Primary 5 and 6 word problems, and the skills the younger years are built on. The bar model, how to spot it in the question, and the mistake children make. A booklet you can print. Free.
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