The table shows the number of students in a class who play on a school team and/or are members of a school club: what is the missing number in the table?   3  5  25  27

This one is very simple.

create a simple equation to find out

15 = 12 + x

subtract 12 from each side

x= 3

the missing number is 3

or, to be even simpler, take the total, subtract the total from the number you know and the difference is the missing number.

hope I helped

ALSO, we can check going down vertically. 3 + 10 =13. this is the total listed at the bottom.