How to measure the speed of sound in air?

STEP ONE:

Let  you and your friend stand as far away as possible from a large reflecting wall and clap your hands rapidly at a regular rate.  

STEP TWO:

Adjust this rate until each clap just coincides with the return of an echo of its predecessor, or until clap and echo are heard as equally spaced.

STEP THREE:

Use a stopwatch to find the time between claps, t. Make a rough measurement of distance to the wall, s. Thus the speed of sound, v = 2s/t