Because the mass of electrons are extremely smaller than protons and neutrons, it has no effect on atomic mass. Therefore, it's not included.
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