I remember flying with a Captain once who loved riddles...."I walk with 4 legs in the morning, 2 during the day, and 3 at night" Blah, blah, blah. All day long.. Then he'd throw a fit when I refused to play his inane game. The one truth I found was that he NEVER gave enough true information to make a logical progression to the answer that he hid like his own private toy.
A) Is this plane flying through a uniform atmosphere? If so, what is the density? Is it even earth's atmosphere or some hypothetical vacuum where Newtonian physics alone would apply? Are we to assume no Coriolis force whatsoever?
B) What balloon? What altitude (100,000' or 3')? Is it a Helium, hydrogen, or a water balloon released by a low flying plane three feet above sea level onto a moving vehicle to splash and then continue on precisely 75 miles over the next hour and a half as a limp piece of rubber on the back of a flatbed truck? For that matter, what if it were released 30' below sea level into the Dead Sea, and was transported by sea currents. In that case, is the water balloon full of fresh water or sea water? It would matter.
C) There are about a dozen other potential variables which come to mind, however I believe you are probably looking at the earth's rotation as the apparant means of displacement. However if that's the case, assuming (which is always a mistake) it's a uniform atmoshere, where was the balloon released? At the equator the earth is rotating at just over 1,000 mph, however at the poles, there is no measurable movement at all. Which opens another question regarding the tilt of the earth's axis. And we were assuming no Coriolis force, right?
Anyone ever see "Princes Bride"? I love that movie.
Regardless, based on the remarkably limited amount of information you gave, and making several enormous assumptions, my answer is zero... (That's my ranking of this question, not my assessment of the wind speed.)
