An urn contains $X$ white balls and $120-X$ red balls while $X$ is a random variable satisfying $\mathbb{E}X=30$. We take one ball from this urn with equal probabilities.
What is the probability that this ball is white?
I have a solution that says that $\frac{1}{4}$ is the answer since this is the expected value of the random variable $\frac{X}{120}$ but I don't understand why is that true.
Please any help.