We know that drift velocity, ${\mathit{v}}_{\mathit{d}}=\frac{\mathit{I}}{\mathit{n}\mathit{A}\mathit{q}}$
Where $\mathrm{I}$ is the current, $\mathit{n}$ is charge density, $\mathit{q}$ is charge of electron and A is cross-sectional area.

${\mathit{v}}_{\mathit{d}}=\frac{27}{9\times {10}^{28}\times 2.5\times {10}^{-7}\times 1.6\times {10}^{-19}}$
${\mathit{v}}_{\mathit{d}}=7.5\times {10}^{-4}\mathrm{m}∕\mathrm{}\mathit{s}$
or ${\mathit{v}}_{\mathit{d}}=0.75\mathrm{mm}{\mathrm{}\mathit{s}}^{-1}$
This is the required average drift velocity.