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Let the shopkeeper buys x number of books.

Cost of 1 book = 8000/x       ................(1)

In the second case, cost of 1 book =  Rs. 8000/ (x + 40)  ........(2)

According to the condition,

(8000/x)  - (8000/ (x + 40)) = 10

On simplifying you will get a quadratic equation, x² + 40x - 32000 = 0

On solving the quadratic equation you will get the number of books as 160 and 200.

So, initially he bought 160 books.