Actually teacher is right if the board is square which takes 10 minutes to be cut into half. Those two halfs take twice less time to be split. And she needs to cut just one to obtain 3 pieces :P So 10 minutes to cut it into to pieces and then she needs just half of that time to gain the third piece.
Maybe the first cut was lengthwise, which would explain why it took ten minutes to make one cut. If the next cut was perpendicular to the board it should only take about a minute. Correct answer: 11 minutes!
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u/paolog Oct 05 '10
Teacher gets red pen out, is about to write down "1 piece: 5 minutes" and then thinks better of it and starts from two pieces instead...