Answer: $ \large\frac{2v_0 (v_1+v_2)}{2v_0+v_1+v_2}$

Average velocity $\overline{v} = \large\frac{2 v1 v2}{v1+v2}$

In this case, a body travels first half a distance with velocity $v_0 \rightarrow v1 = v_0$.

For the 2nd half, average velocity $v2 = \large\frac{1}{2} $$( v_1+v_2)$

$\Rightarrow \overline{v} = \large\frac{2 v_0 \frac{v_1+v_2}{2}}{v_0 + \frac{v_1+v_2}{2}}$

$\Rightarrow \overline{v} = \large\frac{2v_0 (v_1+v_2)}{2v_0+v_1+v_2}$