Costing

A transport company operates a regular delivery service from its warehouse at point A to a destination at point B, the total annual kilometres covered (including 100 outward and return journeys) being 18,000 km’s per 10-ton vehicle

For the outward journey the vehicle is always fully loaded and in addition there is a regular demand for return loads of 400 tons per vehicle per annum. The standard charge to customers is 24 cents per ton/km. The costs of operating this service per vehicle are as follows:-

Vehicle fixed charges $5,400 per annum
Drivers’ wages, including normal overtime $10,260 per annum
Vehicle running costs 33 cents per km

The company is willing to pay a bonus to drivers of up to 30% of any additional profit for obtaining additional return loads.

(a) What annual profit would be earned per vehicle without any additional return loads?

Answer:

Annual cost per vehicle
Fixed charges $5,400
Drivers’ wages $10,260
Running cost 18,000 x 33 cents $5,940
Total Cost ($21,600)

Ton/km’s
Income
100 outward journeys = 9,000km’s x 10tons 90,000
Return journeys = 400 tons x 90 km’s 36,000
Total 126,000 x 24 cents $30,240

Annual profit per vehicle 8,640

Below my questions:
I am completely confused by the unit.
1)why the income is calculated by 100 outward journeys = 9,000km’s x 10tons
why the answer uses 9000km to times 10tons?
2)why return journey calculates per journey i.e (9000/100)
it’s not per ton per km

Thank you very much.