Hello, my name is Steven. I need some help with step by step explanations for some microeconomics practice problems I am on. I am having trouble grasping how to solve them and some assistance would be greatly appreciated! Thank you!

The table of costs below is for a firm producing milk in a perfectly competitive market. (They line up)

Quantity

0

20

40

60

80

100

120

total cost

250

300

380

520

670

870

1120

1. If the market equilibrium price of a gallon of milk is$10, what is the marginal revenue associated with producing the 65th gallon ofmilk?
2. If the market equilibrium price of a gallon of milk is$10, what level of output maximizes profit for thisfirm?
3. Use an expression to define thefirm's profit when it is maximizing its profit at a price of$10?