A person bought 50 pens for Rs. 50 each. He sold 40 of them at a loss of 5%. He wants to gain 10% on the whole. Then his gain percent on the remaining pens should be = ?

Answer: Option D

According to the question,
Cost price of pen = Rs. 50
Cost price of 50 pen = Rs. 50 × 50
= Rs. 2500
To gain 10% overall sold at = Rs. 2750
Now, 40 pen sold at 5% loss
∴ Selling price of 40 pens
= 40 × 47.5
= Rs. 1900
Remaining 10 pens sold to get overall profit of 10% at = Rs. 850
selling price of 1 (remaining pens)
$$=\frac{850}{10}=\text{Rs}\text{. 85}$$
Cost price of 1 pen = Rs. 50
Profit % of remaining pens
$$\begin{array}{rl}& \text{ = }\frac{35}{50}\times 100\\ & =70\%\end{array}$$