Formulate but do not solve the problem. The Coffee Shoppe sells a coffee blend made from two coffees, one costing $3/lb and the other costing $4/lb. If the blended coffee sells for $3.60/lb, find how much of each coffee is used to obtain the desired blend. Assume that the weight of the blended coffee is 100 lb. (Let x and y denote the number of pounds of the $3/lb and $4/lb coffees, respectively.)