determining speed of sound

In an experiment to determine the speed of sound in air, a student bangs two blocks of wood together when standing 125m away from a high wall. The student hears the echo from the wall within one second of banging the blocks together. When he bangs the blocks repeatedly at a certain rate, he can no longer hear the echoes.

(a) Explain why he cannot hear the echoes when he bangs at a particular rate??

Is it because the rate at which he bangs is simultaneous to that at which he recieves the echo.

(b)The lowest rate of banging for which the echoes cannot be heard is 81 bangs perminute. Calculate a value for the speed of sound.

Is it 2*125*1.35 = 337.5m/s