Question P4–P5–P6 Suppose and swap
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Mrs Lim buys 24 stamps at the post office, a mix of 50-cent stamps and 80-cent stamps. In total she pays $15.60. How many 80-cent stamps did she buy?
Answer
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Suppose everything is one kind, then swap one at a time until the total is right.
In this one. Assume the cheaper option all the way through, see how far short of the real total that falls, then divide by the fixed amount one swap adds — the same move as chickens and cows.
Two kinds of thing are mixed together with a known total count and a known total of something else, such as legs or sweets, and neither kind's count is given on its own.
Children lose track of whether a swap adds to the total or takes away from it. Compare the starting total with the target first, then swap in the direction that closes the gap.
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