Quadratic Equations

let num of books be x

so cost of each book is 80/x

if he had bought 4 more books then number of books=x+4

so

cost of each book in that case=80/(x+4)

ATQ,

80/X-[80/(X+4)]=1

Solving we get

(x+20)(x-16)=0

so

x=16, as negative num can't be taken for number of books.

so, he bought 16 books.