Explanation:

- A dozen books are priced at Rs120, so the price for one book is Rs120 divided by 12, which is Rs10.

- A 15% discount on each book means you save 15% of Rs10, which is Rs1.50.

- Thus, the discounted price per book becomes Rs10 - Rs1.50 = Rs8.50.

- You have Rs51 to spend.

- To find out how many books you can buy with Rs51, divide Rs51 by the discounted price per book, i.e., Rs8.50.

- Rs51 divided by Rs8.50 equals 6.

Here's the breakdown of options:

- Option 1, 8 books: 8 books would cost Rs68, which exceeds Rs51.

- Option 2, 5 books: 5 books would cost Rs42.50, under Rs51.

- Option 3, 6 books: 6 books would cost Rs51, which is exactly your budget.

- Option 4, 7 books: 7 books would cost Rs59.50, which exceeds Rs51.