If two tuning forks A and B are sounded together, they produce 4 beats per second. A is then slightly loaded with wax, they produce 2 beats when sounded again. The frequency of A is 256. The frequency of B will be 

$n_A=$  Known frequency  $=256Hz,n_B=?$
$x=4bps,$  which is decreasing after loading  $\left(i.e.x\downarrow \right)$
Also known tuning fork is loaded so  $n_A\downarrow$
Hence  $n_A\downarrow -n_B=x\downarrow \stackrel{ }{\to }$  Correct  …….. (i)
$n_B-n_A\downarrow =x\downarrow \stackrel{ }{\to }$  Wrong  …….. (ii)
 $\Rightarrow n_B=n_A-x=256-4=252Hz$ .