Tuning fork F₁ has a frequency of 256 Hz and it is observed to produce 6 beats/second with another tuning fork  F₂. When F₂ is loaded with wax, it still produces 6 beats/second with F₁. The frequency of  F₂ before loading was

n_A = Known frequency = 256 Hz, n_B = ?
x = 6 bps, which remains the same after loading.
Unknown tuning fork F₂ is loaded so n_B$\downarrow$
Hence n_A – n_B$\downarrow$  = x        ... (i)         $\to$         Wrong
            n_B$\downarrow$  – n_A = x        ... (ii)        $\to$        Correct
$\Rightarrow$ n_B = n_A + x = 256 + 6 = 262 Hz.