John paid a sum of money for purchasing 20 pens, which he recovered in full when he sold 10 of them. What was his percentage of profit or loss per pen?  Choose the correct choice from the following choices.
A) 100 % B) 150 % C) 90 % D) 80 % E) 95 %

Rationale: a 100 % if the sum he paid whilst purchasing 20 pens = a , then the cost price of each pen = a / 20 . since the amount he got whilst selling 10 pens is also = a then the selling price of each pen = a / 10 . since selling price > cost price , he made a profit . profit per pen = selling price - cost price = a / 10 - a / 20 = a / 20 . profit percentage per pen = profit per pen / cost per pen x 100 = ( a / 20 ) / ( a / 20 ) x 100 = 100 %
Answer: A