What do you do with that count? Is it like lightning and thunder, divide by five? Or divide by something else because it’s a blast not a sound.
Technically, blast waves vary a lot, but the sound of the blast wave travels at about a constant speed.
Sound travels about 340 meters or 372 yards per second, about 3 seconds per kilometer or 5 seconds per mile.
If a nuke detonates 30 miles from your location, the light/thermal heat will arrive almost instantly. The sound/shock wave will not arrive for roughly 2.5 minutes.
30 miles x 5 sec/mile = 150 seconds = 2.5 minutes
Here is a video of 5 men standing at ground zero, as an airburst nuclear weapon detonates above them at 10,000 ft., or 1.9 miles. However, when you see the flash, it is about 14 seconds before the blast wave sound hits them. So I suspect that the nuke was launched in an upward trajectory, and was closer to 3 miles high when it detonated.
http://www.youtube.com/watch?v=BlE1BdOAfVc
In practical terms, when a nuke goes off, nobody is going to be thinking about math for quite some time, so just remembering a count of seconds is good enough.
As a final note, it was always said that we should remember the four primary products of a nuclear detonation: blast, heat, radiation, and paperwork.