Suppose that you are studying a standard normal random variable represented by $X$∼$N(0,1)$. You are curious about the expected value of $X$ given that $X$ is greater than $0$. Can you determine what is $E[X∣X>0]$ ?
Source: puzzledquant.com
My approach: How do I get f(x) for this X|X>0? Like I understand we have to integrate from 0 to \infinity xf(x), but I am unable to find the f(x) for this?