(c) When source and observer both are moving in the same direction and observer is ahead of source, then apparent frequency
$v^{'} = \frac{v - v_{o}}{v - v_{s}}v = \frac{v - \frac{v}{6}}{v - \frac{v}{4}} = \frac{\frac{5v}{6}}{\frac{3v}{4}}v = \frac{10}{9}v$