Question P6 Constant total
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Container P and Container Q held marbles in the ratio 9 : 7. After 20 marbles were poured from Container P into Container Q, the ratio became 7 : 9. How many marbles were in Container P at first?
Answer
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Money moves between two people, so their total never changes.
In this one. Pouring between the containers keeps their combined total fixed; because both ratios contain 16 parts altogether, one part has the same value before and after.
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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