Every whole number from 1 to 50 is placed in the correct region of the diagram. How many numbers belong in the region marked ?

The ? region holds even multiples of 3 (multiples of 6) that are not perfect squares: 6, 12, 18, 24, 30, 42 and 48 — seven numbers, because 36 is a square. 8 keeps 36, 16 counts every multiple of 3 up to 50, and 25 counts every even number.