The number density of free electrons in a copper conductor is $8.5 \times 10^{28} \cdot m ^{-3}$. How long does an electron take to drift from one end of a wire $3.0\, m$ long to its other end? The area of crosssection of the wire is $2.0 \times 10^{-6}\, m ^{2}$ and it is carrying a current of $3.0\,A$.