Sound waves transfer

Ultrasonic,infrasonic,and audible waves travel through a medium with speeds $V_u$,$V_i$,and $V_a$ respectively. Then:

$A$ sound wave of frequency $160 \,Hz$ has a velocity of $320 \,m/s$. When it travels through air, the particles having a phase difference of $90^{\circ}$ are separated by a distance of: (in $cm$)

The temperature at which the speed of sound in air becomes double of its value at $27^\circ C$ is ... $^\circ C$

At what temperature (in $^\circ C$) will the speed of sound in air be $3$ times its value at $0^\circ C$ (in $^\circ C$)?

When a tuning fork vibrates,the waves produced in the fork are