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Let the list price of the book be  Rs. x  
Now total budget  be Rs.1200  
No. of books purchased  = 1200/x 
If the list price is increased by 10 Rs then new price = Rs.(x+10) 
1200/(x+10)
ATQ 1200/x-1200/(x+10)=10
1200(10/(x²+10x)) =10 
x²+10x-1200=0
Solving the equation we get x =-40,30
cost cannot be negative hence  list price of the book is Rs.30