By selling a book for Rs 258, a bookseller gains 20%.
Question:

By selling a book for Rs 258, a bookseller gains 20%. For how much should he sell it to gain 30%?

Solution:

Selling price of the book $=$ Rs. 258

Gain $\%=20 \%$

Since, C.P $=\left[\frac{100}{(100+\text { Gain } \%)} \times S . P\right]$

$=\left[\frac{100}{(100+20)} \times 258\right]$

$=\frac{25800}{120}$

$=$ Rs. 215

Let the bookseller sells it for Rs. $x$

So, $S . P=\left[\frac{(100+\text { Gain } \%)}{100} \times C . P\right]$

$x=\frac{100+30}{100} \times 215$

$=\frac{130 \times 215}{100}$

$=$ Rs. $279.50$

Therefore, the bookseller must sell the book at Rs 279. 50 to make 30\% profit.