If 11 like basketball and 16 like hockey, you get a total of 27, but if 2 said both then you don't want to double count them, so a total of 25 students answered. If 25 total students answered but 2 liked both, then 23 only liked one.
The answer to (a) is 5 students did not like either sport
The answer to (b) is 23 students liked exactly one sport.