I was looking at the answer for Why P=MC instead of MR=MC? The willingness of the customer to pay should decrease with each additional unit because of the law of diminishing returns, which makes me think it should be MR=MC in equilibrium.
TR = P*Q = 400Q-4Q^2.
MR = dTR/dQ = 400-8Q.
400-8Q=140.
260=8Q.
Q = 32.5 units.

P = 400 - 4Q
= 400 - 4*32.5
= 400 - 130
= $270

Maybe I don't know when to use price instead of marginal revenue. What am I missing?

5. profit maximization and shutting down in the short run suppose that the market for polos is a competitive market. the following graph shows the daily cost curves of a firm operating in this market. 0 2 4 6 8 10 12 14 16 18 20 50 45 40 35 30 25 20 15 10 5 0 price (dollars per polo) quantity (thousands of polos) mc atc avc for each price in the following table, calculate the firm's optimal quantity of units to produce, and determine the profit or loss if it produces at that quantity, using the data from the previous graph to identify its total variable cost. assume that if the firm is indifferent between producing and shutting down, it will produce. (hint: you can select the purple points [diamond symbols] on the previous graph to see precise information on average variable cost.) price quantity total revenue fixed cost variable cost profit (dollars per polo) (polos) (dollars) (dollars) (dollars) (dollars) 12.50 135,000 27.50 135,000 45.00 135,000 if the firm shuts down, it must incur its fixed costs (fc) in the short run. in this case, the firm's fixed cost is $135,000 per day. in other words, if it shuts down, the firm would suffer losses of $135,000 per day until its fixed costs end (such as the expiration of a building lease). this firm's shutdown price—that is, the price below which it is optimal for the firm to shut down—is per polo.

Grant, inc., acquired 30% of south co.’s voting stock for $200,000 on january 2, year 1, and did not elect the fair value option. the price equaled the carrying amount and the fair value of the interest purchased in south’s net assets. grant’s 30% interest in south gave grant the ability to exercise significant influence over south’s operating and financial policies. during year 1, south earned $80,000 and paid dividends of $50,000. south reported earnings of $100,000 for the 6 months ended june 30, year 2, and $200,000 for the year ended december 31, year 2. on july 1, year 2, grant sold half of its stock in south for $150,000 cash. south paid dividends of $60,000 on october 1, year 2. before income taxes, what amount should grant include in its year 1 income statement as a result of the investment?

Robert black, regional manager for ford in texas and oklahoma, faced a dilemma. the ford f-150 pickup truck was the best-selling pickup ever, yet ford's headquarters in detroit had decided to introduce a completely redesigned f-150. how could mr. black sell both trucks at the same time? he still had "old" f-150s in stock. in his advertising, mr. black referred to the new f-150s as follows: "not a better f-150. just the only truck good enough to be the next f-150." this statement represents ford's of the new f-150.