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

Frequency of the source = 100 $\pm$ 5 = 105 Hz or 95 Hz.
Second harmonic of the source = 210 Hz or 190 Hz.
As the second harmonic gives 5 beats/sec with sound of frequency 205 Hz, the second harmonic should be 210 Hz.
$\Rightarrow$ Frequency of the source = 105 Hz.