A student stands a distance L from a wall and claps her hands. Immediately on hearing the reflection from the wall she claps her hands again. She continues to do this, so that successive claps and the sound of reflected claps coincide. The frequency at which she claps her hands is f. What is the speed of sound in air?

The distance of the student from the wall is L

After hearing the reflection she claps her hand again

A complete cycle is the distance to the wall and back to the boy, so the total distance travel then is 2L, then the wave length is 2L

λ=2L

So the frequent is f

Then using wave equation

v=f λ

Since our λ=2L

Then, v=f*2L

v=2fL

The speed of sound in air is 2fL