One other thing I noticed. Is that as the ratio of the initial amount of gas the station has to the max amount each car can request (in this problem it's 2) gets large the average number of cars serviced tends towards $\dfrac {GS_{init}}{Req_{max}}$ as you'd expect. I'm a bit surprised that the average number of cars wasn't just 100/25=4 myself but that's the nature of the uniform distribution.

Usually you'd be able to model the sum of uniforms as a Normal rv but the numbers are so low in this problem it's not a very good fit.

Good luck. I hope all this has at least shed some light on the problem.