You're right that the question is how long would it take to cut another board, but then you assumed the total time should include the previous board, which is not the case. The question is about the time it takes to cut ONE board into three pieces, which would require two cuts.
Am I the only one who first wondered why it took Marie so long just to saw one board? 10 minutes? What the fuck is she doing with her time? Masturbating?
The fact that there are people commenting about how the teacher was correct (and assuming they're not trolling) is evidence that we need better quality mathematics education.
The only way this would be accurate is if the wood was a perfect square board. If was cut in half in 10 min and then each half could be cut in half in half the time. 1=10, 2=15, 3=20 (cuts in amount of time). It didn't say the 3 pieces had to be the same size. The picture on the right might just be decoration because the problem above does invole squares. Let's give the teacher the benefit of the doubt please.
That's not how any of this works.
Cut it twice = 3 pieces = 20mins
Thats wrong tho