I remember the same type of joke about a guy jumping up and down on a man-hole cover shouting "44..44..44..44..44". A guy approaches and asks what the other man is doing. The jumper stops, and says give it a try.
When the second man starts jumping, the other pulls the cover off the man-hole causing the second man to fall in.
He replaces the lid and starts jumping on it shouting "45..45..45"