D=100-2p, p is the price in dollars of the product and D is the number of products that can be sold at that price.
What price should be set in order to set in order to sell 75 items?
D=100-2p P=price D=products sold in this case, the number of products you would want to sell is 75 so-> 1) 75=100-2p 2) 75+2p=100 3) 100-75=2p 4) 25=2p 5) 25/2=p 6) p=12.50