The time between the successive crusts will give the time period of the wave.
So, the time period, T of the given wave is 0.2 s.
The frequency of a wave is the reciprocal of its time period. That is,
Frequency, ν = 
= 
= 5 Hz
Thus, the frequency of the given travelling periodic wave is 5 Hz.
The plastic rod is made of insulator (plastic), so it does not allow charges moving from one sphere to another. This means that all the electrons given to sphere A will remain on sphere A.
The number of electrons initially given to sphere A is
, and since the charge of 1 electron is
, the net charge left on sphere A after the removal of the rod will be
