My son has been given these and he can't work out the missing number (X) . Neither can I! Can anyone tell me what they are and especially how they got the answer? Thanks very much.

X 3 5 2 7 10

X 2 7 10 13 9

X 6 10 12 8 11.
By the way, he as given twenty PAGES of maths problems to work out. They were handed out on a Wednesday and due to be handed in on a Monday. He's 12 years old (in grade seven). Does anyone else think it's a bit excessive? Thanks again.
My son's gone to bed now but I think it just said " find the missing number". Also, he wasn't given any time in class for this - it was all expected to be done at home. By the way, my wife is a teacher ( grades 1 and 2 but she is trained to teach all grades) and she was very angry at the amount - I think roughly six to twelve questions per page.