Explanation

(c) As $\frac{1}{f} = (\mu - 1)\left( \frac{1}{R_{1}} - \frac{1}{R_{2}} \right)$
$\therefore\frac{1}{20} = (1.5 - 1)\left( \frac{1}{\infty} - \frac{1}{R} \right)$
$\frac{1}{20} = \frac{- 1}{2R},R = - 10\ $cm
Refraction from rarer to denser medium
$- \frac{\mu_{1}}{u} + \frac{\mu_{2}}{v} = \frac{\mu_{2} - \mu_{1}}{R},\ $where $u = \infty,v = f$
$\therefore 0 + \frac{1.5}{f} = \frac{1.5 - 1}{10} = \frac{1}{20},f = 30\ cm$