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

To do this, the time it takes a sound to travel a measured distance must be recorded. Either the distance must be large (preferably over 50 metres) or the time recorded on an electronic timer (to eliminate errors due to reaction time). You should be able to describe a method of measuring the speed of sound in air.

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15.3 cos 65 degrees trig