An outlet store receives a shipment of model solar systems that will be offered at a discounted price of $39 apiece. The model solar systems were originally sold for $50 each. What is the discount percentage?

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.