two drivers began their journey with the same amount of petrol in their cars at the same time . the only difference is that the first driver � s car could drive 4 hours in that amount of petrol and the second one could drive 5 hours . however , they only drove for some time and found that the amount of petrol that was left in one of the cars was 4 times the petrol left in the other one . for how long had they driven at this point of time ?