Four beats per second are heard when an unknown tuning fork and a 240 Hz tuning fork are struck simultaneously. This same unknown tuning fork and a 250 Hz tuning fork are struck. Six beats are heard. Its exact frequency?

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.