Q43 of 82 Page 4

A shopkeeper buys some books for ₹ 80. If he had bought 4 more books for the same amount, each book would have cost ₹ 1 less. Find the number of books he bought. (CBSE 2012)

Let the number of books bought be x. Then,


Cost of x books = Rs80


Cost of one book


If the number of books bought is x + 4, then


Cost of one book


Cost of one book is reduced by Re 1,


So,






⇒ x2 + 4x = 320


⇒ x2 + 4x – 320 = 0


⇒ x2 + 20x – 16x – 320 = 0


⇒ x(x+20) – 16 (x+20) = 0


⇒ (x - 16) (x + 20) = 0


⇒ x = 16 or x = - 20


As books cannot be negative.


So, x = 16


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