A merchant bought 749 pens. He sold 700 of them for the price he paid for 749 pens. He sold the remaining pens at the same price per head as the other 700. The percent gain on the entire transaction is

Let the cost price of each pen be "p". Hence total cost price of the pens is 749p. He sold 700 pens for 749p. Selling price per pen is $$\frac{749}{700}p\ or\ 1.07p$$

Now selling price of the remaining 49 pens is 1.07p(49) or 52.43p.

Hence total selling price is 749p+52.43p = 801.43p

Total profit made isc 801.43p-749p =52.43p

Profit percentage $$\frac{52.43p}{749p}\times\ 100=7\%$$

Hence net profit percentage is 7%