You and a friend sign up for a health club. it costs $50 to join, plus $20 a month. however, the first 100 customers can sign up for $30, plus $20 a month. your friend is customer 100, and you are 101.
if the accumulated cost for each membership were graphed, what would it look like? what would the difference be between your accumulated cost and your friend's accumulated cost at any given month?