A 12-pound mixture of coffee that contains both decaffeinated and regular coffee costs $56. The decaffeinated coffee costs $4 per pound, and the regular coffee costs $5 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 (a) Write a system of equations whose solution gives the amount of each type of coffee in the mixture. Select the correct choice below and fill in the answer boxes to complete your choice. (Type integers or decimals.) OA x + y = 4x + 5y = OB. 4x + y =​