In a guitar, two strings A and B made of same material are slightly out of tune and produce beats of frequency 6 Hz. When tension in B is slightly decreased, the beat frequency increases to 7 Hz. If the frequency of A is 530 Hz, the original frequency of B will be: 

524 Hz

Difference of f_A and f_B is 6 Hz

If tension decreases, f_B decreases and becomes f'_B .

Now, difference of f_A and f'_B = 7 Hz (increases)

So, f_A > f_B f_A – f_B = 6 Hz

f_A = 530 Hz ; f_B = 524 Hz (original)