The charge (in dollars) for painting a sports car is given by C(x, y) = 4x² + 5y² - 4xy + √x, where x is the number of hours of labor needed and y is the number of gallons of paint and sealant used. Find the following.
(a) The charge for 10 hours and 5 gal of paint and sealant
(b) The charge for 15 hours and 10 gal of paint and sealant
(c) The charge for 20 hours and 20 gal of paint and sealant