$\Rightarrow$$3\left( {\frac{v}{{4{l_1}}}} \right) = 2\left( {\frac{v}{{2{l_2}}}} \right)$;
where $l_1$ and $l_2$ are the lengths of closed and open organ pipes
hence $\frac{{{l_1}}}{{{l_2}}} = \frac{3}{4}$
$\left(t_{0}\right.$ represents the instant when the distance between the source and observer is minimum)
