The label has been scratched off a tuning fork and you need to know its frequency. From its size, you suspect that it is somewhere around 250 Hz. You find a 250-Hz tuning fork and a 270-Hz tuning fork. When you strike the 250-Hz fork and the fork of unknown frequency, you hear a modulation (fluctuation of the amplitude) at a frequency of 5 Hz. When you strike the unknown with the 270-Hz fork, you hear a modulation at a frequency of 15 Hz. What is the unknown frequency