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