Frequency of tuning fork A,
vA = 512 Hz
Now, frequency of tuning fork B may be-
VB = vA ± 5 = 512 ± 5 = 517 or 507 Hz
As on loading B, frequency decreases
If vB = 517 Hz, if might reduced to 507 Hz, given again a beat of 5 Hz.
If vB = 507 Hz, reduction will always increase the frequency of beat, so, VB = 517 Hz