I am not certain how you got -80, and I could be very wrong, but I think the shutdown price would be where the quantity is equal to 5. That would make the price 4 and TR = 4*5 = 20 subtract from that a total cost of $35 and you get -15.
"but I think the shutdown price would be where the quantity is equal to 5. That would make the price 4 and TR = 4*5 = 20"
I agree that the shutdown price occurs at a quantity of 5, but why is the price = $4? I get a shutdown price also of 5. So I get a total revenue of $25 and a profit of -10. Di I miss something?
"I thought that I read something somewhere that said that marginal cost is equal to price."
I think the book says that Marginal Cost = supply price in a short-run competitive market. Thus in order to maximize profit the price must equal marginal cost. In parts B and C I got a shutdown price of $5 occuring at a quantity of 5. The table says that at a quantity of 5, var_cost=25 and fix_cost=10 for a total cost of 35. Part C says that at the shutdown price, the total revenue = total variable cost (which is 25 on the according to the table).