Answer is 15 miles per hour
TeX rendering has been iffy at best on this site for the past few days, at least in my experience. I've attached a solution below.
Make f(x) = y
y = (15x + 50)/(x + 5)
Swap the x and y and make y the subject again
x = (15y + 50)/(y + 5)
x(y + 5) = 15y + 50
xy + 5x = 15y + 50
xy - 15y = 50 - 5x
y(x - 15) = 50 - 5x
y = (50 - 5x)/(x - 15)
So inverse function f^-1(x) = (50 - 5x)/(x - 15)