Stepped costs in budgeting

Aoife co manufactures headphones and has developed a new handset H. The maximum production capacity is 150,000 units and the management accountant is preparing an annual flexible budget.

Units 100,000. 120,000. and 150,000
Material costs $700000. $840,000 and $1050000 respectively.
Labour costs $750000. $900000 and 1125000 respectively.
Incremental fixed costs are $60,000 each for the three units.

a) assuming the budgeted figures are correct what would the flexed total production be if production is 90% of maximum capacity ?

The answer is 150000*90%=135000
Stepped supervisors costs=135000/15000=9 steps
I understand it till here but i don’t understand the the stepped costs= 9 steps * $42000 per step= $378000
i dont understand how they got the $42000

Could you please help? 🙂