Question

A Shopkeeper buys a number of books for Rs. 1200. If he had bought 10 more books for the same amount, each book would have cost Rs 20less. How many books did he buy?

Answer 1

Let the shopkeeper buys X books for Rs. 1200

So, each book costs Rs. (1200/X)

Now he bought 10 more books at the same price

So, new no of books is (X+10)

Now each book costs 1200/(X+10)

If he bought 10 more books at the same price then each book have Rs. 20 less.

So, the equation to solve this problem is

(1200/X)-(1200/(X+10))=20

Or, X^2+10X-600=0

Or, X^2+(30-20)X-600=0

Or, X^2+30X-20X-600=0

Or, X(X+30)-20(X+30)=0

Or, (X+30)(X-20)=0

So, X= -30 and X= 20

Number of books can't be negative so, we consider X=20

Answer: Number of books he bought is 20