Question P5–P6 Constant total
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Farah and Wei Ling had a total of 180 trading cards. Farah gave of her trading cards to Wei Ling. Wei Ling then gave of her trading cards to Farah. In the end, each of them had the same number of trading cards. How many trading cards did each of them have at first?
Answer
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Money moves between two people, so their total never changes.
In this one. The total stays fixed at 180, so fix the equal ending at 90 each first, then undo the two gifts in reverse order.
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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