A sound wave has a frequency of 4 kHz and a wave length of 40 cm. The time taken by the sound wave to travel a distance of 3.2 km is:

It is given that,

Frequency of sound wave, f = 4 kHz = 4000 Hz

Wavelength of sound wave, λ = 40 cm = 0.4 m

We have to find the time taken by the sound wave to travel a distance of 3.2 km,  
$$d=3.2\times10^{m} m$$
Velocity of sound is given by :
$$\upsilon=f\times\lambda$$
$$\upsilon=400Hz\times0.4 m$$
$$\nu=1600 \frac{m}{s}$$

Velocity of wave is also given by distance travelled per unit time.
$$t=\frac{d}{\upsilon}$$
$$t=\frac{3.2\times10^{3}m}{1600\frac{m}{s}}$$
t = 2 seconds.