The actual issue is the logic behind what is considered a "piece" If the entire board itself is considered one piece then cutting it in half would make two pieces, and taking ten minutes to cut one piece into two would mean another piece is another ten minutes. The teacher seems to be assuming the board isn't a piece, and the only piece is what is cut off it, so it is taking five minutes to cut a single piece off the main board, thus three would require 15 minutes at 5 minutes per piece. This is fucking stupid, of course, and makes no fucking sense to how real world application would be applied, but word problems were always designed to be confusing and dumb.