Note: All tyres are used up to their full strength.
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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