I think this should have been D2.

For all D to satisfy the inequality, the square root of N must also satisfy it. Specifically N-20 <= sqrt(N).

Squaring each side and rearranging yields: N^2 - 41N + 400 <= 0. This means 16 <= N <= 25.

Manually checking each composite integer 16, 18, 20, 21, 22, 24, 25 finds N=21 (1 <= 3 and 7 <= 9) and N=25 (5 <= 5 <=13) are the only solutions.