Question

Brian rented a book from the book shop.The shop charges $2 for the first day and $3 from the second day onwards.Brian took more then 2 days to return the book and he paid $35 while returning the book.How many days did he keep the book with him?

Answer 1

36=n(n-1)/2=35+1,because it begun from2
72=n(n-1)
n=8=days
1+2+3+...=n(n-1)/2