In the analogy BD : 8 :: CF : ?, which number replaces the question mark?

Each letter is turned into its place and the two places are multiplied. B is 2nd and D is 4th, and $2\times 4=8$. C is 3rd and F is 6th, so $3\times 6=18$. The trap 9 adds the places instead of multiplying, but 2 + 4 = 6 and not 8. 15 comes from counting F as the 5th letter and 24 from counting C as the 4th, so both rest on a miscount.