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?

• jushmk: 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.