18 copies of a book were sold for Rs.1260/- thereby gaining cost price of 3 copies. The cost price of a copy is:

Rs.60

Profit = S.P. of 18 - C.P. of 18

C.P. of 3 = S.P. of 18 - C.P. of 18

C.P. of 21 = S.P. of 18

    C.P.        :         S.P.

      18        :         21

       ⇓                    ⇓ ×60

       ⇓ ×60         1260 (given)

 (18 × 60)

C.P of 18 articles = 18 × 60

C.P. of each article = Rs.60/-