A car has 4 tyres and 1 spare tyre. Each tyre can travel a maximum distance of 20000 kilometers before wearing off. What is the maximum distance the car can travel. You are allowed to change tyres (using the spare tyre) unlimited number of times.
Note: All tyres are used up to their full strength.
Divide the lifetime of spare tire into 4 equal part i.e., 5000 and swap it at each completion of 5000 Kms distance.
Let four tyres be A, B, C and D and spare tyre be S.
5000 KMs: Replace A with S.