In the bay area , the golden gate charges a toll of $15. the bay bridge charges $7.5. you use each bridge some number of times , for a total of 11 uses . if you spend $105 in total on tolls . how many times did you use each bridge ?

Golden gate (G) uses = $15*G
Bay bridge (B) uses = $7.50*B
Together =>
Equation 1 : $15*G+7.50*B=$105
Equation 2: G+B=11
Solve system of equations
15G + 7.50B = 105 Eq1
1G + 1B = 11 Eq2

Multiply eq2 by -7.50
15G + 7.50B = 105
-7.5G - 7.5B = -82.5
————————————
7.5 G + 0B = 22.5
Solve for G
G= 22.5/7.5=3

Now plug G into equ1 or equ2
I used equ2
G+B=11
3+B=11
Solve of B
B=11-3=8

So they used golden gate 3 times
And bay bridge 8