A) 251 Hz
B) 254 Hz
C) 261 Hz
D) 256 Hz
Correct Answer: A
Solution :
There are 5 beats between A and B, therefore the possible frequencies of A are 251 or 2611 (that is \[256\pm 5\]) Hz. When the prong of A is filed, its frequency becomes greater than the original frequency. If we assume that the original frequency of A is 261, then on filling its frequency will be greater than 261. The beats between A and will be more than 5. But it is given that the beats are reduced to 2, therefore 261 is not possible. Therefore, the required frequency must be 251 Hz. This is true, because on filing the frequency may increase to 254, giving 2 beats with B of frequency 256 Hz.
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