Answer
(a) Electrons have been added to the plastic rod.
(b) $1.25\times 10^{11}$ electrons were added to the rod.
Work Step by Step
(a) Since it is normally quite easy for electrons to move, we can assume that it is the electrons which are transferred when there is a transfer of charge. In this case, we can assume that electrons have been added to the plastic rod.
(b) Each electron has a charge of $-1.6\times 10^{-19}~C$. We can find the number of electrons that were added.
$\frac{-20\times 10^{-9}~C}{-1.6\times 10^{-19}~C} = 1.25\times 10^{11}~electrons$
$1.25\times 10^{11}$ electrons were added to the rod.
