A net charge of the sphere is the overall charge of the sphere. If it says that there are excess electrons, it means that the net charge is negative. Also, the quantity of charge of sub particles are very small so that must be 10 to the power of negative 17. So, I think the net charge is -4.8×10⁻¹⁷ Coulombs.
Then, we use the electrical charge of an electron which is equal to 1.60217662×10⁻¹⁹ C/electron. Therefore,
-4.8×10⁻¹⁷ Coulombs * 1 electron/-1.60217662×10⁻¹⁹ C = 299.592
Therefore, there are approximately 300 excess electrons.
