Explanation

(b) $\frac{1}{f_{air}} = \left( \mu_{g} - 1 \right)\left( \frac{1}{R_{1}} - \frac{1}{R_{2}} \right)\ \ \ \ \ \ldots(i)\ $
$and\ \frac{1}{f_{water}} = \left( \frac{\mu_{g}}{\mu_{w}} - 1 \right)\left( \frac{1}{R_{1}} - \frac{1}{R_{2}} \right)\ \ \ \ \ \ \ \ldots(ii)$
Dividing Eq. (i) by Eq. (ii) we get
$\frac{f_{water}}{f_{air}} = \left( \frac{\mu_{g} - 1}{\mu_{g}/\mu_{w} - 1} \right) = \frac{\left( \frac{3}{2} - 1 \right)}{\left( \frac{3/2}{4/3} - 1 \right)\ }f_{air}$
$= 4f_{air} = 4 \times 10$
$f_{water} = 40\ cm$