Question P5–P6 Repeated identity
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Nadia and Farid are stamp collectors. Each of them has 150 stamps in his or her album, made up only of 20-cent stamps and 60-cent stamps. Nadia has 18 more 60-cent stamps than Farid. Farid has 40 sixty-cent stamps in his album. Each 60-cent stamp weighs 3 g and each 20-cent stamp weighs 2 g.
Answer
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One length or count, described two ways in the same question.
In this one. Because both albums contain the same number of stamps, cancel the matching stamps: only the 18 swaps from a 20-cent stamp to a 60-cent stamp can change the value.
A shape or count is described from two viewpoints in the same question, such as one edge measured as a number of long pieces and again as a number of short pieces.
Children treat the two descriptions as separate quantities and add them. Both describe the same quantity, so the two must be equal.
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