24,000 units
Contribution margin is selling price minus variable costs. The remaining amount goes to fixed costs, and anything left after that is pre-tax income.
So we find the total contribution margin (CM per item times number of items) and then subtract the fixed costs. In this case, we can do this by solving for x:
15x-320,000= 40,000 (add 320,000 to both sides)
15x= 360,000
x=24,000