Two tuning forks x and y produce tones of frequencies 256 Hz and 262 Hz respectively. An unknown tone sounded with x produces, beats. When it is sounded with y the number of beats produced is doubled. The unknown frequency is

Let the number of beats heard with ‘x’ is ‘P’ then unknown frequency

The number of beats produced Is doubled with ‘y’ then

From the above two cases, it is clear that 
262 – 2P = 256 + P  or 256-P=262-2P

∴P = 2 or  6
∴ unknown frequency ; n = 256 – p = 258 Hz or 250Hz