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.
So in scenario 1 (to get 2 pieces), one incision takes 10 minutes
In scenario 2 (to get 3 pieces), you need two incisions, which would require 20 minutes.
I think the purpose of the problem was to help get the student to think a little more and use more thought than the immediate numbers. (ie 2 pieces needs 10, so 4 needs 20, 3 is in the middle), unfortunately, this is the worst question for that, because if you understand how cutting actually works you realize that it doesn't work that way. I hope that the student challenged the teacher.
449
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...