# Thread: Quick question

1. ## Quick question

Show that , if X has a nonnegative integers as values , then E(X)=Sum P(X>n)

thanks in advance

2. Hello,

$P(X>n)=\sum_{j=n+1}^\infty P(X=j)$

Then reverse the summation order.

You may look at a similar problem here : http://www.mathhelpforum.com/math-he...screte-rv.html