A source of sound gives 5 beats per second when sounded with another source of frequency 100 second^-1. The second harmonic of the source, together with a source of frequency 205 second-1 gives 5 beats per second. What is the frequency of the source ?

Frequency of source
$$\begin{gathered} =100\pm 5=105 Hz\text{ or }95\mathrm{Hz} \\ \mathrm{As} \mathrm{the} \mathrm{second} \mathrm{harmonic} \mathrm{gives} 5 \mathrm{beats}/\mathrm{secoud} \mathrm{with} \mathrm{sound} \mathrm{of} \mathrm{frequency} 205 \mathrm{Hz}, \mathrm{the} \mathrm{second} \mathrm{harmonic} \mathrm{should} \\ be ( 205 \pm 5) Hz . So frequency of second harmonic is given by \\ 2f = 200 Hz or 210 Hz . So f = 100 Hz or 105 Hz. \\ \mathrm{Frequency} \mathrm{of} \mathrm{the} \mathrm{source} = 1\mathrm{O}5\mathrm{Hz} \end{gathered}$$