A retired auto mechanic hopes to open a rustproofing shop. Customers would be local new-car dealers. Two locations are being considered, one in the center of the city and one on the outskirts. The central city location would involve fixed monthly costs of $6,870 and labor, materials, and transportation costs of $30 per car. The outside location would have fixed monthly costs of $4,200 and labor, materials, and transportation costs of $40 per car. Dealer price at either location will be $90 per car.

At what volume of output will the two sites yield the same monthly profit?