$$ \left.\langle\overline{\mathrm{v}}\rangle=\frac{\text { Displacement }}{\text { time }} \right\rvert\, $$ (Let displacement be I) $$ \begin{aligned} & =\frac{\ell}{\left(\frac{\ell}{\mathrm{V}_3}+\frac{\ell}{\mathrm{V}_2}+\frac{\ell}{\mathrm{V}_1}\right) \frac{1}{3}}=\frac{3}{\frac{1}{\mathrm{~V}_1}+\frac{1}{\mathrm{~V}_2}+\frac{1}{\mathrm{~V}_3}}=\frac{3}{\frac{1}{11}+\frac{1}{22}+\frac{1}{33}} \\ & =18 \mathrm{~m} / \mathrm{s} \end{aligned} $$