Two friends hit the accelerator of their respective cars at the same time. Both of them have the same amount of petrol in their respective cars at the time they start. One can drive for 4 hours and the other can drive for 5 hours.

After driving for a while, they analyse that the amount of petrol left in one of their cars is 4 times that of the petrol left in other car.

Can you calculate how long they have been driving their respective cars ?