Answer: 9.9999872 C
Explanation: In to answer this question we have to use the charge of the electron, that is eqaul to -1.6*10^-19 C.
After that, we have to calculate the charge given by 8.0*10^13 electrons, then we an additional charge of: 8.0*10^13 * -1,6*10^-19 C=1.28*10^-5C
Finally the net charge of the metal sphere, initially charged by +10C is:
10C-1.28*10^-5C=9.9999872 C