A school-boy standing some distance in front of a high cliff claps his hands once and hears the echo after 0.80 s.
a) Calculate the distance between the boy and the cliff.
b) The boy then walks 40 m closer to the cliff and repeats the clap. What time interval should he now expect between the clap and the echo? (Take the speed of sound in air to be 340 m s⁻¹.)