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.
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