A man allows a discount of 10% on a book whose marked price

A man allows a discount of 10% on a book whose marked price is Rs. 40. What is the cost price so that the profit is 20%?

Given that , marked price = Rs. 40 , discount = 10% , profit = 20%
Let C.P. of article = Rs. y
According to question ,

y ×	120	=	40 × 90
	100		100

⇒ y =	40 × 90	= Rs. 30
	120

We can find required answer with the help of given formula :
Here, D = 10%, r = 20%, M.P. = Rs. 40, C.P. = ?

M.P.	=	100 + r
C.P.	=	100 − D

40	=	100 + 20
C.P.	=	100 − 10

C.P. =	40 × 90	= Rs. 30
	120