Discussion
A rectangular paper of 20 cm × 8 cm is folded 3 times. Each fold is made along the line of symmetry, which is perpendicular to its long edge. The perimeter of the final folded sheet (in cm) is
Answer: A
The perimeter is 18 cm.
Each fold halves the current long edge, and after each fold the long edge has to be checked again:
- 20 cm × 8 cm: fold across the 20 cm edge to get 10 cm × 8 cm.
- 10 cm × 8 cm: the long edge is still 10 cm; fold to 5 cm × 8 cm.
- 5 cm × 8 cm: now the long edge is 8 cm; fold to 5 cm × 4 cm.
The perimeter is \(2(5 + 4) = 18\) cm.
21 cm is the trap: it halves the 20 cm side all three times, giving 2.5 cm × 8 cm, and ignores that the long edge changes after the second fold.
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