18 : 8 :: 20 : ? Complete the number analogy.

The number is broken into its prime factors and those factors are added up, each one counted as often as it appears. 18 is $2\times 3\times 3$, so the total is 2 + 3 + 3 = 8; 20 is $2\times 2\times 5$, so the total is 2 + 2 + 5 = 9. The trap 7 counts each different prime only once, 2 + 5, but doing that to 18 gives 2 + 3 = 5 and not 8. 10 is half of 20, where half of 18 is 9, and 6 is the count of the divisors of 20 — a count 18 shares, so it cannot be the rule.