This is probably an elementary exercise, but my head spins trying to think about it. Assume that on Earth we see something 1 billion light years away (i.e. light took 1 billion years to reach us). To simplify, assume no relative proper motion, although the distance will change due to universe expanding. How far apart are the Earth and the source now and how far apart were they when the light was emitted ( 1 billion years ago)?