15.18 Two sitar strings A and B playing the note ‘Ga’ are slightly out of tune and produce beats of frequency 6 Hz. The tension in the string A is slightly reduced and the beat frequency is found to be reduced to 3 Hz. If the original frequency of A is 324 Hz, what is the frequency of B?

Frequency of string A, νA=324 Hz 
Frequency of string B=νB
 Beats frequency, n = 6 Hz
Initial beat's frequency:
νB=330 Hz or 318 Hz
The frequency is directly proportional to the square root of tension.
So the frequency decreases with a decrease in the tension in a string.
Hence, the frequency of B cannot be 330 Hz.
 νB=318 Hz