Two tuning forks A and B give 4 beats per second. The frequency of A is 256 Hz. On loading B slightly, we get 5 beats in 2seconds. The frequency of B after loading is

n_A = Known frequency = 256 Hz, n_B = ?
x = 4 beats per sec which is decreasing (4 bps to $\frac{5}{2} \mathrm{bps}$) after loading (i.e. x$\downarrow$)
Unknown tuning fork B, is loaded so n_B$\downarrow$
Hence  n_A – n_B$\downarrow$  = x$\downarrow$        ... (i)        $\to$        Wrong
           n_B$\downarrow$  – n_A = x$\downarrow$        ... (ii)         $\to$      Correct 
$\Rightarrow$ n_B = n_A + x = 256 + 4 = 260 Hz.