Explanation

(c) For Pfund series, $\frac{1}{\lambda_{s}} = R\left( \frac{1}{5^{2}} - \frac{1}{(\infty)^{2}} \right) = \frac{R}{25}$
$\lambda_{s} = 25/R$
$\frac{1}{\lambda_{l}} = R\left( \frac{1}{5^{2}} - \frac{1}{6^{2}} \right) = R\left( \frac{36 - 25}{25 \times 36} \right)$
$\lambda_{l} = \frac{25 \times 36}{11R}$
$\therefore\ \frac{\lambda_{l}}{\lambda_{s}} = \frac{25 \times 36}{11R} \times \frac{R}{25}$
$= \frac{36}{11}$