A supplier of computer games has two warehouses, A and B, which are located in different parts of the country. Warehouse A has 90 copies of a new computer game in stock, whereas B has 60 copies. Customers in town C request 50 copies and those in town D request 80 copies. The costs of delivering a copy to customers in towns C and D are shown in the table.

(a) If the supplier wishes to arrange delivery to minimise costs, show that the problem can be written as a linear programming problem involving two variables.

(b) Draw the feasible region for the problem in part (a) and hence determine the optimal delivery arrangements.