Which number completes the analogy 36 : 9 :: 24 : ?

The second term counts how many whole numbers divide the first. 36 is divided by 1, 2, 3, 4, 6, 9, 12, 18 and 36 — nine of them. 24 is divided by 1, 2, 3, 4, 6, 8, 12 and 24 — eight. The trap 7 drops the number itself from the count, which on the first pair would have given 8 and not 9. 6 drops both the 1 and the number, giving 7 the first time, and 12 is merely half of 24, against 18 for 36.