Two tuning forks A and B vibrating simultaneously produce 5 beats. Frequency of B is 512. It is seen that if one arm of A is filed, then the number of beats increases. Frequency of A will be

After filling frequency increases, so  nA decreases (↓). Also it is given that beat frequency increases (i.e., x ↑)  Hence nA↓ – nB  = x↑        ... (i)       →        Correct           nB  – nA↑ = x↑        ... (ii)       →        Wrong⇒ nA = nB + x = 512 + 5 = 517 Hz.