Question P6 Constant total
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Ken and Mei's piggy banks held money in the ratio 3 : 1. After Ken transferred $30 to Mei's piggy bank, the ratio became 1 : 3. How much money did Ken have at first?
Answer
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Money moves between two people, so their total never changes.
In this one. Because a transfer keeps the combined total fixed, the same total of 4 parts before and after means each part has the same value, so Ken's drop from 3 parts to 1 part is exactly the $30 transferred.
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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