The speed of sound in a medium depends on

A source of sound is at one end of a hollow tube. An observer at another end hears two distinct notes after a time interval of $1$ $ \mathrm{s}$. If velocity of sound in air is $340$ $ \mathrm{m}$ $ \mathrm{s}^{-1}$ and in metal is $3740$ $ \mathrm{m}$ $ \mathrm{s}^{-1}$, then the length of pipe is,

A machine gun is mounted on an armoured car moving with a speed of $20{ \mathrm{m} \mathrm{s}}^{-1}.$ The gun can point against the direction of motion of the car. The muzzle speed of the bullet is equal to the speed of sound in the air i.e., $340{ \mathrm{m} \mathrm{s}}^{-1}.$ The time difference between bullets actually reaching and the sound of firing reaching at a target $500 \mathrm{m}$ away from a car at the instant of firing is