Question

If you shout at a cliff wall that is 440 m away and the air temperature is at 25 °C, how long will it take before you hear your echo?

Answer 1

The speed of sound at T=25°C is Vs=346 m/s. So the sound has to reach the cliff and return back to you so the path it needs to travel is s=2*440 m = 880 m.
Since the speed of sound is constant s=Vs*t, and t= s/Vs=880/346=2.54335 s. You will hear the echo after t=2.54335 s after you shouted.