Question

Anna has 12 bills in her wallet, some $5 and some $10. The total value of the bills is $100. How many of each bill does Anna have? is this how i do this?
x+y=12
5x+10y=100
y=12-x
5x+10(12-x)
5x+120-10x=100
5x-10x=2-5x=20
x4
y8

Looks good to me.

Matt