Answer: -3.4 . 10⁻¹⁸ C
Step-by-step explanation:
Each electron carries a charge of -1.6 . 10⁻¹⁹ C, which is the smallest discrete charge possible in the universe.
If we add 21 electrons to the insulated sphere, the added charge will be as follows:
Q = 21 . 1.6. 10⁻¹⁹ C = -3.4. 10⁻¹⁸ C.