The wavelengths of two sound notes in air are $\frac{40}{195} \mathrm{~m}$ and $\frac{40}{193} \mathrm{~m}$. Each note produces 9 beats per second separately with a third note of fixed frequency. The velocity of sound in air in $\mathrm{m} / \mathrm{s}$ is

• Correct answer
  A

  360
• B

  320
• C

  300
• D

  340

Solution

Here, $\quad v_1=\frac{v}{\left(\frac{40}{195}\right)}$ $\begin{array}{ll}\text { and } & v_2=\frac{v}{\left(\frac{40}{193}\right)} \\ \Rightarrow & v_1=\frac{v(195)}{40}\end{array}$ and $\quad v_2=\frac{v(193)}{40}$ According to given condition
Adding Eqs. (i) and (ii) $\begin{aligned} 18=\frac{v}{40}(195-193) & =\frac{2 v}{40} \Rightarrow 18=\frac{v}{20} \\ \text { or } \quad v & =360 \mathrm{~m} / \mathrm{s} \end{aligned}$

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