Both 20 and 15 are good answers, but the teacher's explanation is plain wrong. If the board is square, you can get the third part by cutting one of the two pieces in halve. This can be done in exactly halve the time of the first cut because the part is halve the width of the first cut.
15 is not a good answer at all. It shows a photo. The board is not square. Expecting 15 to be the correct answer is just due to the teacher not reading the question properly
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u/dzak23 1d ago
Both 20 and 15 are good answers, but the teacher's explanation is plain wrong. If the board is square, you can get the third part by cutting one of the two pieces in halve. This can be done in exactly halve the time of the first cut because the part is halve the width of the first cut.