there are 24 students with laptops plugged into joe's cafe's outlets.

a). If all the cell phones were charging with 5V DC output and 2.1 A in addition to all of the laptops charging with 19 V DC output and 1.6 A, how much would it cost Joe to charge all of your devices at "Cup o' Joe's" for two hours if he is billed by the power company at a rate of $0.13 per kWh?

b). If Joe nets $5 profit from each of your fancy hot beverages, how many would he have to sell in order to cover this device charging expense?