# Alternate expression for P(X+Y|X)?

I am trying to solve this problem (no one has yet answered, though the comment by Xi'an says it is simple!)

entropy: is H(X+Y) = H(X) + H(X+Y)|X) true?

In writing out the entropy expression, one of the probabilities is $P(X+Y|X)$. Does this expand or simplify to any other expression that could be used?