The lower numbers are multiples of 5 as mentioned before,

The circle contains the numbers:
2, 3, 4, ?, 5, 10, 15, 20, with the number 5 in the center.
If we analyze the numbers in the upper half (2, 3, 4, ?), and the lower half (5, 10, 15, 20):
5 × 1 = 5 and 5 × 2 = 10..