When you purchase 40 songs, you get 24 free song downloads.
Simplify the ratio: (40/24)/(8/8) = 5/3
You must purchase 5 songs to get 3 free song downloads.
Now find how much songs you must purchase to get 18 free downloads. Multiply 6 to numerator & denominator:
(5/3)(6/6) = 30/18
To get 18 free song downloads, you must purchase 30 songs.
~
