Which one will replace the question mark ?

Key idea: the central number equals the sum of the four numbers surrounding the circle.

• First circle: 1 + 2 + 3 + 4 = 10

• Second circle: 8 + 1 + 3 + 5 = 17

• Third circle (missing center): 9 + 1 + 4 + 6 = 20

Therefore the missing number is 20.