Let the original price of the book be= x
and let the actual no of books be= y
By increasing the price of a book by rs.10, present price = x+10
number of books can be bought for the new price = 1200/(x -10) = (1200-10x)/x
so, the equation is,
x+10 (1200-10x / x) = 1200
if we solve the equation we will get x=30
So, the list price of the book is Rs 30.