<em>b</em><em> </em>for <em>books</em><em> </em>read. b=1x + 1. the amount of months he's been in the club goes for x and +1 because he already read a book before he joined. if you do easy math, he's been there for 8 months because he read 9 books. i probably got the equation backwards but this works both ways imo.
100×16.60=1,660
Sam cost
1,660+1,660×0.06=1,759.6
