Jenny runs a birdhouse manufacturing company. the number of birdhouses she can make with each number of laborers is shown in the table below. The price of birdhouses drops from $20 to $18 when the number of birdhouses produced goes to 20 or above. In this situation, what is the marginal revenue product for jenny to go from four to five workers? This is the table format (x, y) x= labor y=birdhouses (0,0) (1,6) (2,11) (3,15) (4,18) (5,20) (6,21)