The absolute difference between two frequencies is referred to as beat frequency. That is,
fb = |f2-f1|
In this case, the frequency of unknown fork must be between 240 Hz and 250 Hz. Then,
4 = x - 240 => x = 240+4 = 244 Hz
This is true since;
6 = 250 - 244 (given in the second case).
Therefore, the unknown frequency is 244 Hz.
