how do I use the substitution method with this word problem?

Question

how do I solve, "there is a group of 8 kids that each want a snack from Steak n' Shake. Fries cost $2 each and onion rings cost $3 each, and the kids spend a total of $18, how many of each did they buy?" I can solve the easily using my own methods but i don't know how to solve this with the substitution method. If you can help please show me how you did it, thank you.

Answer 1

Let R = # rings and F = # fries

R + F = 8

3R + 2F = 18

F = 8 - R

Substitute

3R + 2(8 - R) = 18

R = 18 - 16 = 2

and F = 8 - R = 6