Sienna decides to buy a car. She has enough money saved up for the down payment on the car. The total cost of the car including taxes is $4800. She pays $500 down and then agrees to pay the balance over the next two years in 24 equal payments. Sienna works part time at a bookstore and is paid $12 for each hour she works. Her employer has agreed to let her work extra hours each month over the next two years to earn the additional money she will need for her car payment each month. What is the approximate number of additional hours per month Sienna will have to work over the next two years in order to earn enough extra money to cover her monthly car payment?