Question P6 Repeated identity
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A kite maker joins two rectangular fabric panels, ABDF and BCEF, along a straight seam CDE. AB = 15 cm, AF = 20 cm and BF = 25 cm. Find the length of BC.
Answer
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One length or count, described two ways in the same question.
In this one. One triangle has one area but two usable bases — swap to the base you already know (BF), and the side you are hunting (BC) turns into that base's height.
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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