Frequency is defined as the number of waves per second. In this machine 25 waves pass in one second.
We need to calculate the number of waves that pass a particular point during one second.
During 2 seconds -25 waves
Therefore in one second - 25/2 = 12.5 waves/s.
1 wave per second has the unit Hertz (Hz)
Therefore answer is 12.5 Hz
