Let total number of books be x
Cost of each book = Rs 60/x
Now if 5 more books had been purchsed i.e. x+5 books had been purchased, then each book would have cost Rs 60/(x+5+) and this would have given a saving of Re 1 on each book.
So, 60/x - 60/(x+5) =1
Solving this we get, 60 x + 60 . 5 - 60x = 1 . x (x+5)
i.e. 300 = x^2 + 5x or, x^2+5x - 300 =0
or, x^2+ 20x - 15x- 300 =0 [ by factorising LHS]
or, x(x+20)- 15 (x+20) = 0
or, (x+20) (x-15) = 0
or, x= -20 or x=15
x being total number of book can not be negative. so x=15.
Total number of book is 15. ANS.
[ check your answer -
If 15 books are purchases, cost of each book = 60/15 = Rs 4.
If 5 more books i.e. 20 books had been purchased for same cost, each book would haave cost
60/20 = Rs 3 which is 1 less thas 4. Theanswer satifies condition of question]