At which number of workers is marginal product

Question

Murphy's Machinery is a profit-maximizing firm with the following production information. The price of machine parts is $8.

0	0
1	10
2	25
3	50
4	80
5	95
6	90

1. At which number of workers is marginal product maximized?
2. Identify and define the economic principle that explains why marginal product eventually decreases.
3. Why would Murphy's Machinery never hire the 6th worker?
4. What is the maximum daily wage Murphy's Machinery would be willing to pay the 3rd worker?