Two people are about to begin an 18000 km trip across Asia in a Land Rover. They know that each tire will last for exactly 12000 km and not one bit more. Since they have a four-wheeled vehicle, can you figure out the least number of spares they need to complete their trip?

Answer 1

Number of tire-km required
= 4 * 18,000
= 72,000
Minimum totoal number of tires
=72,000/12,000
=6
The two that they carry are not spares, so if they carry two extra tires, they do not allow for accidents or defects of manufacturing.

They need to rotate the tires at least three times at equal intervals (6,000 km). Tires denoted by letters in parenthses are not mounted.

ABCD(EF) 0-6,000
(AB)CDEF 6,000-12,000
AB(CD)EF 12,000-18,000 km

To have even wear, they should rotate more often.