I was going to post this under Bad Homework Questions, because I had myself convinced that it was a question without a single answer, but I finally figured out where I was going wrong on it.
However, DS had such an offbeat approach to solving it that I wanted to ask if any mathy people here can tell me why it works or if it's a massive coincidence.
We've been going through the practice SAT on their website, and this was one of the questions:
There are 25 trays on a table in the cafeteria. Each tray contains a cup only, a plate only, or both a cup and a plate. If 15 of the trays contain cups and 21 of the trays contain plates, how many contain both a cup and a plate?
DS looked at it and immediately said 11. I asked him how he got that, and he said that cups are 60%, plates are 84%, which is 144%, so you have to subtract 44% to get back to 100%, and 44/100 equals 11/25, so the answer is 11.
Is there any sense to that?
