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?  Options:
A) Rs.30
B) Rs.25
C) Rs.45
D) Rs.20
E) Rs.10
Let the incorrect cost price be C1 and let the original cost price be C2.   Marked price of book is Rs.30. It is 20% more than C1.   Therefore, (120/100) X C1 = 30   Or C1 = 25.   C1 is more than C2 margin of 25%.   Or C1 = (125/100)C2   Therefore, C2 = (100/125) x 25 = Rs 20   ANSWER:Rs.20
<result>D</result>
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?