The base charge is 35 dollars per day. He rented the car for 3 days, so before you add in the mileage you have to multiply 35 x 3 days for 105 dollars. Then you have to take the remaining charge and divide it by the charge per mile to get the number of miles driven. Lets set up the equation.
d= number of days rented
m= number of miles driven
35(d) + .15(m) = 117.25
35(3) + .15m = 117.25
105 + .15m = 117.25
.15m = 117.25 - 105
.15m = 12.25
m = 12.25/.15
m = 81.67
So he drove 81.67 miles