A retailer who sells television recorders finds that she can sell 50 recorders per month when the price is $400 per unit. For each $5 decrease in price, one additional recorder can be sold. If she purchases each unit for $100 and pays $20 to a technician to adjust and fine-tune each unit when it is delivered, what level of sales will maximize monthly profit?

I've been stuck on this problem for a while and I am stumped. I would appreciate any help on it. Thanks.