(c) $t = \frac{1}{\sin\theta}\sqrt{\frac{2h}{g}\left( 1 + \frac{K^{2}}{R^{2}} \right)}$
$\Rightarrow \frac{t_{S}}{t_{D}} = \sqrt{\frac{1 + \left( \frac{K^{2}}{R^{2}} \right)_{S}}{1 + \left( \frac{K^{2}}{R^{2}} \right)_{D}}}$
$= \sqrt{\frac{1 + \frac{2}{5}}{1 + \frac{1}{2}}} = \sqrt{\frac{14}{15}}$