Aditya went to a stationery shop to buy some Parker pens. Gel pens cost Rs 300 each while fountain pens cost Rs 400 each. Aditya spent a total of Rs 3600 on pens. If he had bought as many fountain pens as the number of gel pens he actually bought and vice-versa, he would have saved an amount equal to half the cost of one pen of one of the two types. Find the total number of pens he bought. (a) 8        (b) 9       (c) 10          (d) 12