A car idling for 38 hours will use its supply of gas. The same car, when driven at 60 miles per hour uses three and half times as much fuel per hour as it does idling, If the engine has been idling for 10 hours and the car is then run at 60 miles per hour, how much longer will the car run before it uses up all of its fuel?

Exactly 8 HOURS.

The calculation:-

1) Idling = 1 Tank = 38 hours
2) Driving = 1 Tank = 38/3 1/2 = 76/7 hours

3) 38 hours less 10 hours = 28 hours

4) Proportion :-

76/7 for 38 How much for 28 ?

Calculate this will give you 8 Hours exactly.