A 17 -pound mixture of coffee that contains both decaffeinated and regular coffee costs $ 60. The decaffeinated coffee costs $3 per pound, and the regular coffee costs $4 per pound. (a) Let x be the weight of the decaffeinated coffee and y be the weight of the regular coffee. Write a system of equations whose solution gives the amount of each type of coffee in the mixture.
(b) Use the method of substitution to solve the system of equations.