When a tuning fork A of unknown frequency is sounded with another tuning fork B of frequency 256 Hz, then 3 beats per second are observed. After that A is loaded with wax and sounded, the again 3 beats per second are observed. The frequency of the tuning fork A is

It is given that 
n_A = Unknown frequency = ?
n_B = Known frequency = 256 Hz
x = 3 bps, which remains same after loading 
Unknown tuning fork A is loaded so n_A$\downarrow$
Hence n_A$\downarrow$ – n_B  = x        ... (i)        $\to$       Correct
            n_B  – n_A$\downarrow$ = x        ... (ii)       $\to$        Wrong
$\Rightarrow$ n_A = n_B + x = 256 + 3 = 259 Hz.