because there are 5 tires available and only 4 tires are used at a time each tire travels 4/5 of the distance
lets use 1000 km for example:
1000/5 = 200 km
The first 200 km we use tires 1, 2, 3, 4
The second 200 km we use tires 1, 2, 3, 5
The third 200 km we use tires 1, 2, 5, 4
The fourth 200 km we use tires 1, 5, 3, 4
The fifth 200 km we use tires 5, 2, 3, 4
breaking it into 5ths you can see each tire is used 4/5 of the time
so: 30000 x 4/5 = 24000 km on each tire
