12 copies of a book were sold for ₹1800/- thereby gaining cost price of 3 copies. The cost price of a copy is:

₹120

Profit = S.P. of 12 - C.P. of 12

C.P. of 3 = S.P. of 12 - C.P. of 12

C.P. of 15 = S.P. of 12

    C.P.        :         S.P.

      12        :         15

       ⇓                    ⇓ × 120

       ⇓ × 120         1800 (given)

 (12× 120)

C.P of 12 articles = 12 × 120

C.P. of each article = Rs.120/-