I am confused about how to solve this question, because the profit is the same at both the 6th and 7th units. Profit = 180 for these, MR=MC at the 7th unit. the correct answer is 50, which is the 6th unit's average variable cost. can someone explain why the answer is not 54.2, which is the 7th unit's average variable cost? is there a rule where you always calculate it for the first point where profit is maximized?