In Exercises 15-22, assume that relative maximum and minimum values are absolute maximum and minimum values.

Maximizing profit. A concert promoter produces two kinds of souvenir shirt. Total revenue from the sale of x thousand shirts at $18 each and y thousand at $25 each is given by

R ( x , y ) = 18 x + 25 y .

The company determines that the total cost, in thousands of dollars, of producing x thousand of the $18 shirt and y thousand of the $25 shirt is

C ( x , y ) = 4 x 2 − 6 x y + 3 y 2 + 20 x + 19 y − 12.

How many of each type of shirt must be produced and sold to maximize profit?