• # question_answer 28) By selling 100 pens, a shopkeeper gains the selling price of 20 pens. Find his gain percent? A)  10%                            B)  15%         C)  30%                            D)  25%

(d): Let C.P of one pen = x Let gain % be p% $\Rightarrow S.P.\left( one\text{ }pen \right)=x\times \left( \frac{100+p}{100} \right)$ On sale of 100 pens, S.P of 100 pens $=100x\left( \frac{100+p}{100} \right)$ For 100 pens, gain $=S.P-C.P=x\left( 100+p \right)-100x$ This gain = S.P (20 pens) $=20x\times \left( \frac{100+p}{100} \right)$ $\Rightarrow px=\frac{x\left( 100+p \right)}{5}$ $\Rightarrow 5px=100x+px$ $\Rightarrow 4px=100x$ $\Rightarrow 4p=100=p=20\,or\,25%$