Good question. lol. I don't remember.
All I know is that you can rack about 80 of them in about 2 minutes if you do 5 quick, with 2 second rest. I usually just breathe in and out, then normally while doing the push up.
However, logically, breathe in when doing easy, breathe out when giving efforts. So when going down, breathe in, going up breathe out. Sort of like a bench press. When you go down, you breathe in, hopefully. And when pushing it back up, you breathe out. Or do a quick combination of in and out when it gets hard while doing the effort motion.