Explanation
If you know the altitude of the plane, along with the delay between the time the plane passes overhead and the time you hear the jet engines, you can determine the speed of sound by dividing the altitude by the time delay. Observing a sprinter’s speed in reacting to a starter’s pistol, the last choice might be another way to measure the speed of sound. However, the pistol and sprinter might be too close to each other to create a measurable delay; what’s more, the sprinter’s reaction time will depend on other factors as well.