Answer
$1.9 \times 10^{-13}$%
Work Step by Step
Because the comb is charged to 3.0µC, it lost 3.0µC electrons.
∵$e = 1.6^{-19} C$
∴$M_{lost-electrons} = 3.0µC \div 1.6^{-19} C \times 9.1 \times 10^{-31}kg = \frac{3.0 \times 10^{-6}C}{1.6^{-19} C} \times 9.1 \times 10^{-31}kg = 1.7 \times 10^{-17} kg = 1.7 \times 10^{-14} g$
∴The change of mass = $\frac{1.7 \times 10^{-14} g}{9.0g} \times$ 100% = $1.9 \times 10^{-13}$%
