enny runs a birdhouse manufactruing company. the number of birdhouses she can make with each number of laborers is shown in the table below

labor - birdhouses

0- 0

1-6

2-11

3-15

4-18

5-20

6-21

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