Let the cost of x no. of books Rs.1200 price for each book = 1200/x If he had bought 10 more books for the same amount each book would have cost Rs 20 less A.T.Q1200/x -1200/x+10=20. [as per according to the given question]1200(1/x -1/x+10)=20. [we take 1200 as common][x+10-x/x(x+10)]=1/60. [by taking 1200 on RHS]10×60=x^2+10x. [by cross multiplication]x^2+10x-600=0x^2+30x-20x-600=0. [by splitting]x(x+30)-20(x+30)=0. [by taking x and -20 common](x-20)(x+30)x=20 and x=-30. ( but it is not possible as we don't have negative numbers of books)therefore x=20so, number of total books =20

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

Understanding the problem:
We need to find the number of books the shopkeeper bought. We are given the total cost of the books and the information that if 10 more books were bought for the same amount, the cost per book would have been reduced by Rs.20.

Let's assume:
Let's assume that the shopkeeper initially bought 'x' books.

Calculating the initial cost per book:
Since the shopkeeper bought 'x' books for Rs.1200, the initial cost per book can be calculated by dividing the total cost by the number of books:
Cost per book = Total cost / Number of books
Cost per book = 1200 / x

Calculating the revised cost per book:
If the shopkeeper had bought 10 more books for the same amount, each book would have cost him Rs.20 less. This means the revised cost per book would be:
Revised cost per book = (Total cost - Rs.20) / (Number of books + 10)
Revised cost per book = (1200 - 20) / (x + 10)

Setting up the equation:
According to the given information, the initial cost per book is equal to the revised cost per book:
1200 / x = (1200 - 20) / (x + 10)

Solving the equation:
To solve the equation, we can cross-multiply:
1200(x + 10) = x(1200 - 20)

Expanding and simplifying the equation:
1200x + 12000 = 1200x - 20x

The x terms cancel out, leaving us with:
12000 = -20x

Dividing both sides by -20:
x = -12000 / -20
x = 600

Therefore, the shopkeeper bought 600 books.

Conclusion:
The shopkeeper bought 600 books.