The C.P of 10 pens is equal to the S.P of 5 pens. Find his gain % or loss%?
Choose one.
A) 50 %  B) 40 %  C) 30 %  D) 60 %  E) 20 %

Rationale: 10 cp = 5 sp 10 - - - 5 cp gain 100 - - - ? = > 50 % ( gain ) answer : a
Answer: A