STICKELERS

Below is a magic square where the sum of the numbers in each row, column and diagonal is the same. What number replaces the question mark?

The answer is 16.

Notice that 36 + 8 must equal 20 plus the number in the middle. Why? Both sets share the number that goes in the upper left corner. That makes the middle number 24. The second row is 8, 24 and 40, which total 72. So the third row will be 36, 16 and 20 (which total 72).