C= cards and S = sets and G = goal and X = the amount needed to reach the goal
72 C = 1 S
G = 15S
and 15 S is equal to 15 multiplied by 72C
15(72C) =G
1080C = G
if he already has 216 cards then
216C + X = 1080C
you should be able to figure out the rest :) Good luck and PM for additional questions