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

Rs.90

Profit = S.P. of 17 - C.P. of 17

C.P. of 3 = S.P. of 17 - C.P. of 17

C.P. of 20    =    S.P. of 17

    C.P.        :         S.P.

      17        :         20

       ⇓                    ⇓ × 90

       ⇓ × 90         1800 (given)

 (17× 90)

C.P of 17 articles =  17× 90

C.P. of each article = Rs.90/-