A company produces two models of a surfboard: a standard model and a competition model. If the standard model is produced at a variable cost of $160 each and the competition model at a variable cost of $320 each, and if the total fixed costs per month are $4,000, then the monthly cost function is given by C(x,y) = 4,000 + 160x + 320y where x and y are the numbers of standard and competition models produced per month, respectively. Find C(20,10), C(70,7), and C(40,40). C(20,10) = $ (Simplify your answer.) C(70,7) = $ (Simplify your answer.) C(40,40) = $ (Simplify your answer.)