Question P4–P5–P6 Constant total
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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?
Answer
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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.
One person gives, sells or loses an amount to the other, and nobody outside the two people gains or loses anything.
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.
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