Frequency of sonometer wire $n \propto \frac{1}{l}$.
$\therefore $ $\frac{{n + 4}}{{n - 4}} = \frac{{100}}{{95}}$ or $95\,(n + 4) = 100(n - 4)$
or $95n + 380 = 100n - 400$ or $5n = 780$ or $n = 156$
$(i)$ The source may be moving towards the observer with a velocity of $30\,ms^{-1}$
$(ii)$ The source may be moving towards the observer with a velocity of $33\,ms^{-1}$
$(iii)$ The observer may be moving towards the source with a velocity of $30\,ms^{-1}$
$(iv)$ The observer may be moving towards the source with a velocity of $33\,ms^{-1}$
