What can be complicated for some people is not the math in itself, it's the combination of both the timing which makes you *beh bah buh aaaaaahh shit i'm wrong* and how the question is asked. It's not like "Which of the following is a multiple of 5 ?" it's "How much you need to add to have a multiple of 5 ?" So well I can understand that it may be a bit complicated.