A man sells a book at a profit of 20% . If he had bought it at 20% less and sold it for Rs. 18 less, he would have gained 25% . The cost price of the book is = ?

Answer: Option D

Let the C.P. be Rs. 100
Then,
Profit = 20%
S.P. = Rs. 120
New C.P. = 20% less = Rs. 80
If he gain is 25% then S.P.
$$\begin{array}{rl}& =\text{Rs}\left(\frac{125}{100}\times 80\right)\\ & =\text{Rs}.100\end{array}$$
Difference in S.P.
= Rs. (120 - 100)
=Rs. 20
If difference in S.P. is Rs. 20, then C.P. = Rs. 100
If difference in S.P. is Rs. 18, then
$$\begin{array}{rl}& \text{C}\text{.P}\text{.}=\text{Rs}.\left(\frac{100}{20}\times 18\right)\\ & =\text{Rs}\text{. }90\end{array}$$