Answer
$4.0 \times 10^9 \cdot (-1.6 \times 10^{-19} \, \text{C}) = -0.64 \, \text{nC}$
Work Step by Step
The glass and silk acquire equal and opposite charges due to the conservation of charge. Since electrons have a negative charge, the silk becomes negatively charged, while the glass rod becomes positively charged. The total charge of $4.0 \times 10^9$ electrons is calculated as above.
