A school principal wants to buy some book cases for the school library. She can choose between two types of book case. Type A costs $10 and it requires 0.6 m^2 of floor space and holds 0.8 m^3, of books. Type B costs $20 and it requires 0.8 m^2 of floor space and holds 1.2 m^3 of books. The maximum floor space available is 7.2 m² and the budget is $140 (but the school would prefer to spend less). What number and type of book cases should the principal buy to get the largest possible storage space for books?​