John paid a sum of money for purchasing 30 pens, which he recovered in full when he sold 20 of them. What was his percentage of profit or loss per pen?	Pick one: A) 50 % B) 60 % C) 40 % D) 30 % E) 45 %

Rationale: "a 50 % if the sum he paid whilst purchasing 30 pens = a , then the cost price of each pen = a / 30 . since the amount he got whilst selling 20 pens is also = a then the selling price of each pen = a / 20 . since selling price > cost price , he made a profit . profit per pen = selling price - cost price = a / 20 - a / 30 = a / 60 . profit percentage per pen = profit per pen / cost per pen x 100 = ( a / 60 ) / ( a / 30 ) x 100 = 50 %"
Answer: A