15 copies of a book were sold for Rs.5400/- thereby gaining cost price of 3 copies. The cost price of a copy is:

300

Profit = S.P. of 15 - C.P. of 15

C.P. of 3 = S.P. of 15- C.P. of 15

C.P. of 18 = S.P. of 15

    C.P.        :         S.P.

      15      :         18

       ⇓                    ⇓ × 300

       ⇓ × 300      5400 (given)

 (15× 300)

C.P of 15 articles = 15× 300

C.P. of each article = Rs.300/-