posted by Courtney .
You watch a distant woman driving nails into her front porch at a regular rate of 1 stroke per second. You hear the sound of the blows exactly synchronized with the blows you see. And then you hear two more blows after you see her stop hammering. How far away is she?
It takes the sound two seconds (the elapsed time between strokes 2 apart) to reach the listener. Multiply that time by the speed of sound to get the distance.