The cost price of an book is 64% of the marked price. Calculate the gain percent after allowing a discount of 13%? Choose the correct choice:
A) 31.5 %  B) 35.9 %  C) 37.5 %  D) 39.5 %  E) 30.5 %

Rationale: "let marked price = $ 100 . then , c . p . = $ 64 , s . p . = $ 87 gain % = 23 / 64 * 100 = 35.9 % . b"
Answer: B