Subject: mathqa
Question: the marked price of a book is 20 % more than the cost price . after the book is sold , the vendor realizes that he had wrongly raised the cost price by a margin of 25 % . if the marked price of the book is rs . 30 , what is the original cost price of the book ?
Option: 
A: rs . 20
B: rs . 45
C: rs . 10
D: rs . 30
Answer: A