In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature

An echo is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 ms^-1?

What is the audible range of the average human ear?

What is the range of frequencies associated with

(a) Infrasound?

(b) Ultrasound?