Step cost are tyre replacement cost, which are 300 at the end of every 30,000 miles.
If the car travel less than or exactly 30,000 miles in 2 year, the tyres will not be changed . average cost of tyres per annum = 0.
If a car travels more than 30,000 miles and up to 60,000 miles in 2 year, there will be 1 changed of tyres in the period. Average cost of tyres per annum = 150.
If a car exceed 60,000 miles in 2 year(up to 90,000 miles) there will be 2 tyre changes. Average cost of tyres per annum =300.