The teacher marked "15" as wrong, but they're actually the hero we need! When you cut a board into 2 pieces, you make 1 cut . For 3 pieces? That's 2 cuts . The question is asking about cuts, not pieces! The student brilliantly recognized the pattern (10 min = 1 cut, so 20 min = 2 cuts, thus 15 min = 1.5 cuts... which makes zero sense unless Marie has a quantum saw). Meanwhile, the teacher's answer of "20 minutes" assumes a linear relationship between pieces and time, which is mathematically unsound. This is why we can't have nice things in education.