Let v1 and v2 be the frequencies of the strings A and B, respectively.
Then, v1 = 324 Hz, v2 = ?
Number of beats,
b = 6
v1 = v1 ± b = 324 ± 6
i.e., v2 = 330 Hz or 318 Hz
Since, the frequency is directly proportional to square root of tension, on decreasing the tension in the string A, its frequency v1 will be reduced, i.e., number of beats will increase, if v2 = 330 Hz. This is not so because number of beats becomes 3.
Therefore, it is concluded that the frequency = v2 = 318 Hz, because on reducing the tension in the string A, its frequency may be reduced to 321 Hz, thereby giving 3 beats with v2 = 318 Hz