1 Electron flow
How many electrons pass through a control cross-section of a metallic conductor when the current of $40~{\rm mA}$ flows for $4.5~{\rm s}$?
\begin{align*}
Q &= I \cdot t \\
&= 0.04~{\rm A} \cdot 4.5~{\rm s} \\
&= 0.18~{\rm As} \\
&= 0.18~{\rm C} \\
&={0.18~{\rm C}}\cdot {1\over{1.6022*10^{-19}{\rm C/electron}}} = 1.1 \cdot10^{18}~{\rm electrons}
\end{align*}
\begin{align*}
1.1*10^{18}~{\rm electrons}
\end{align*}