$A$ polythene piece rubbed with wool is found to have a negative charge of $3 \times 10^{-7} \;C$.
$(a)$ Estimate the number of electrons transferred (from which to which?)
$(b)$ Is there a transfer of mass from wool to polythene?

(A) When polythene is rubbed against wool,a number of electrons get transferred from wool to polythene. Hence,wool becomes positively charged and polythene becomes negatively charged.
Amount of charge on the polythene piece,$q = -3 \times 10^{-7} \; C$.
Amount of charge on an electron,$e = -1.6 \times 10^{-19} \; C$.
Number of electrons transferred from wool to polythene $= n$.
$n$ can be calculated using the relation,$q = ne$.
$n = \frac{q}{e} = \frac{-3 \times 10^{-7}}{-1.6 \times 10^{-19}} = 1.875 \times 10^{12}$.
Therefore,the number of electrons transferred from wool to polythene is $1.875 \times 10^{12}$.
$(b)$ Yes. There is a transfer of mass taking place. This is because an electron has mass,$m_{e} = 9.1 \times 10^{-31} \; kg$.
Total mass transferred to polythene from wool,$m = n \times m_{e} = 1.875 \times 10^{12} \times 9.1 \times 10^{-31} \; kg$.
$m \approx 1.706 \times 10^{-18} \; kg$.
Hence,a negligible amount of mass is transferred from wool to polythene.