A sound wave has a frequency of 5kHz  and a wavelength of 50cm. The distance traveled by sound in 4sec (in $\mathrm{km}$ ) will be:

We know that the speed formula in terms of wavelength and frequency is:
Speed (v)= wavelength $\left(\lambda \right)\times$frequency (f).
According to the question given,
Wavelength $\left(\lambda \right)=50 \mathrm{cm}=50/100 \mathrm{m}=0.5 \mathrm{m}$
Frequency $\left(f\right)=5\mathrm{kHz}=5000 \mathrm{Hz}$
Speed $\left(v\right)=f\times \lambda$
$\Rightarrow V=5000\times 0.5=2500 \mathrm{m}/\mathrm{s}$

The distance traveled in $4\sec =$speed x time 

$=2500\times 4$
 $=10000 \mathrm{m}$

The distance traveled in $4\sec =$$10 \mathrm{km}$