Economics Q

Flash Flight produces running shorts in a monopolistically competitive market. It sells no shorts at $50 a pair. With their current advertising budget, each $5 drop in price results in an increase of 20 shorts sold per day. If the firm doubles their advertising budget, they can triple the quantity of shorts sold at each price. The firm’s marginal cost is constant at $5 a pair. What quantity of shorts will maximize profits for Flash Flight? A) 90 per day B) 180 per day C) 270 per day D) 540 per day The anwer is A. I understand how to get 180, i just dont understand the logic of taking half that. Let me know how you get the answer please. Also, how do you know when to take the advertising budget into consideration?