If you get 2 tips up, will you get 2 books?That would be an amazing incentive.
It's exponential. So y=e^x. (where y is the number of books you receive and x is the number of tips).So 2 tips will get you 7 books and 4 tips, for example, will get you 54 books.
Andrew surely you're already read that the exponential function is just the product of repeated multiplications by left wing mathmeticians who need to keep their grant money rising (exponentially)?