Ming plays video games online. When he signed up with the game site, Ming got 200 points.
Ming earns 20 more points for each hour that he plays.

How many hours does Ming have to play to get to 500 points?

Let x have probability generating function gx (s) and let un generating function u(s) of the sequence uo, u1, satisfies p(x > n). show that the (1- s)u(s) = 1 - gx(s), whenever the series defining these generating functions converge.