A bus driving along at $39.6 \mathrm{kmph}$ is approaching a person who is standing at the bus stop, while honking repeatedly at an interval of 30 seconds. If the speed of the sound is $330 \mathrm{~m} / \mathrm{s}$, at what interval will the person hear the horn?

• A

  31 seconds
• Correct answer
  B

  29 seconds
• C

  30 seconds
• D

  the interval will depend on the distance of the bus from the passenger

Solution

$\mathrm{v}=39.6 \mathrm{~km} / \mathrm{hr}=11 \mathrm{~m} / \mathrm{s}, \quad \mathrm{t}_{1}=\frac{\mathrm{d}}{330}$ and $\mathrm{t}_{2}=\frac{\mathrm{d}-11 \times 30}{330}$ Now $\Delta t=t_{1}-t_{2}=1$ now $t_{1}=30 \mathrm{sec} \therefore t_{2}=29 \mathrm{sec}$

Hence, the correct answer is option: 'B'.