If you let the convenience fee equal x, then...
(2×17.25)+(2×1)+(2×x)= 40.50
There are 2 girls so we multiply all the charges by 2. Add them together and you'll get 40.50. Now if we solve for x, we will get the amount of the convenience fee.
2×17.25 + 2×1 + 2x = 40.50
34.50 + 2 + 2x = 40.50
36.50 + 2x = 40.50
2x = 40.50 - 36.50
2x = 4
x = 2
Each girl paid a 2 dollar convenience fee.