Ultrasonic waves are those waves

An engine approaches a hill with a constant speed. When it is at a distance of $0.9 \ km$,it blows a whistle,and the echo is heard by the driver after $5 \ s$. If the speed of sound in air is $330 \ m/s$,the speed of the engine is .... $m/s$.

What is the ratio of the speed of sound in nitrogen gas to that in helium gas at the same temperature of $300 \, K$?

$A$ nuclear bomb exploded $200 \,km$ above the surface of the moon. The sound of the explosion on the moon

$A$ man,standing between two cliffs,claps his hands and starts hearing a series of echoes at intervals of one second. If the speed of sound in air is $340 \, ms^{-1}$,the distance between the cliffs is .... $m$

$A$ man sets his watch by the sound of a siren placed at a distance of $1 \,km$ away. If the velocity of sound is $330 \,m/s$,what is the status of his watch?