a car with 5 tires (four road tires and a spare in the trunk) travelled 30000 km. All five tires we used equally. How many km of wear are on each tire?

30,000 / 5 = 6000 < = = so each tire was used for 6000 km.

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