单选题 A merchant paid $300 for a shipment of x identical calculators．The merchant used two of the calculators as demonstrators and sold each of the others for ＄5 more than the average(arithmetic mean)cost of the x calculators．If the total revenue from the sale of the calculators was ＄120 more than the cost of the shipment，how many calculators were in the shipment?
【正确答案】 E
【答案解析】The merchant paid $300 for a shipment of x calculators, so the average cost, in dollars, per calculator was300/x. The merchant sold (x - 2) of them at the price of 5 + 300/x dollars each, for a total revenue of (x - 2) (5 +300/x) dollars, which was 120 + 300 = 420. Manipulating the equation (x - 2) (5 +300/x) = 420 gives x2 - 26x - 120 = 0 or (x - 30)(x + 4) = 0, which can be solved by factoring. It follows that there were 30 calculators in the shipment.The correct answer is E.