Each of a certain type of tire on a 4 wheel car lasts exactly 10,000 miles. How many spare tires of this same variety must carried so that a trip of 15,000 miles can just be completed.

3 spare tires are needed. This works if you rotate the tires every 5,000 miles.

Tire A, B, C, D (and spare is E F G)

first 5,000 miles you use tire A B C D

second 5000 miles you use tire B C D E

third 5000 miles you use A E F G

Each tires is used no more than twice (10,000 miles).