For question 3, it clearly states two things:

"the total amount of gas available at the stations and in the car is exactly enough for the car to drive around the road once"

and

"the car completes a full circle without running out of gas"

Given the first condition, in your solution, at the exact moment the car completes the drive around the track, it runs out of gas. Therefore, it is impossible to complete it without running out of gas.