The speed of electron is given by $v=2.18\times {10}^6\times \frac{Z}{n} \mathrm{m} {\mathrm{s}}^{-1}$, where, $Z$ is atomic number and $n$ is number of orbit.

Now, for third orbit, ratio of speed of electron is $\frac{v_{\mathrm{H}}}{v_{{\mathrm{He}}^{+}}}=\frac{Z_{\mathrm{H}}}{Z_{{\mathrm{He}}^{+}}}=\frac{1}{2}$

$\Rightarrow \frac{v_{{\mathrm{He}}^{+}}}{v_{\mathrm{H}}}=\frac{2}{1}$