15. Remember that Jamie has to pay her workers $20 per hour, and there are no other production costs. Using the data to the right we know that the approximate marginal cost of the 52^nd unit of output is:

   Marginal Cost is the change in cost due to an increase in output of one unit. When the firm hires a 6^th worker output increases from 47 to 52 so in order to compute the Marginal Cost of the 52^nd unit we need to divide the change in cost ($20, the cost of hiring another worker) by the change in output (5, the increment between 47 and 52). In this case this is the same thing as Wage ÷ MP. So, Marginal Cost = 20 ÷ 5 = 4.