Question 285516
The profit is %20 of the costs. The sales price is $7.50
So {{{Profits = 0.20*Costs}}}

{{{Costs + Profits = SalesPrice}}}
{{{Costs + (0.20*Costs) = 7.50}}}
{{{1.20*Costs = 7.50}}}e
{{{Costs = 6.25}}}

So he makes 1.25 profit per sale.

How many did he sell?

You can't get there from here. The problem doesn't provide enough information to calculate that. You need to know total sales, total costs or total profits -- you need 'something more' in order to figure out 'how many'