X=cost of large snack
Y=cost of small snack
X+Y=60
5X+3Y=220
To solve for y, solve x+y=60 for x. Subtract y from both sides so you're left with x=-y+60. You now plug in -y+60 for x in the second equation.
5(-y+60)+3y=220
-5y+300+3y=220
-2y+300=220
-2y=-80
Y=40
40 small snack packs is the final answer