Lets start by finding the equation.
25 is the constant, .3 is the slope, and 50.2 is the final.
This means that our equation is:
50.2 = .3x + 25
50.2 - 25 = .3x
25.2 = .3x
25.2 / .3 = x
84 = x
This means that Ken travelled 84 miles if his bill was $50.2.
I hope this helps! :)