A single red blood cell contains 4 x 10⁷ electrons. To calculate the total charge of the electrons we have to multiply the number of electrons by the charge per single electron.
The charge carried by a single electron is - 1.6 x 10⁻¹⁹ Coulombs /electron
Number of electrons in a blood cell - 4 x 10⁷ electrons
Therefore total charge of electrons in the red blood cell - 1.6 x 10⁻¹⁹ C x 4 x10⁷
total charge - 6.4 x 10⁻¹² C