In Exercises 29 through 35, assume that the required extreme value is a relative extremum.

A T-shirt shop carries two competing shirts, one endorsed by Tim Duncan and the other by LeBron James. The owner of the store can obtain both types at a cost of $2 per shirt and estimates that if Duncan shirts are sold for x dollars apiece and James shirts for y dollars apiece, consumers will buy 40 − 50x + 40y Duncan shirts and 20 + 60x − 70y James shirts each day. How should the owner price the shirts to generate the largest possible profit?