Let the number of books bought initially be x.
So, original price of the book is 900/x.
Now, the price of the book goes up by Rs.3.
Therefore, price of each book now is 900/x+3
and the number of books brought is reduced by 15===> x-15
Since Amount spent now is also 900 so,
(900+3x)(x-15) = 900
After solving it, We get x= 75 or -60
Now, x cannot be negative
so x= 75
Therefore, original price of book is 900/75 = RS.12
Hope it helps..! ^-^