Car Wheels Problem

Puzzle:

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 upto their full strength.

Puzzle Solution:

25000
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.
10000 KMs: Put A back to its original position and replace B with S
15000 KMs: Put B back to its original position and replace C with S
20000 KMs: Put C back to its original position and replace D with S

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