d
(d) \(\frac{1}{F} = \frac{1}{{{f_1}}} + \frac{1}{{{f_2}}} + \frac{1}{{{f_3}}}\)

\(\frac{1}{{{f_1}}} = (1.6 - 1)\left({\frac{1}{\infty } - \frac{1}{{20}}} \right) = - \frac{{0.6}}{{20}} = - \frac{3}{{100}}\)…\((i)\)

\(\frac{1}{{{f_2}}} = (1.5 - 1)\left({\frac{1}{{20}} - \frac{1}{{ - 20}}} \right) = \frac{1}{{20}}\)…\((ii)\)

\(\frac{1}{{{f_3}}} = (1.6 - 1)\left({\frac{1}{{ - 20}} - \frac{1}{\infty }} \right) = - \frac{3}{{100}}\)…\((iii)\)

\( \Rightarrow \) \(\frac{1}{F} = - \frac{3}{{100}} + \frac{1}{{20}} - \frac{3}{{100}} \Rightarrow F = - 100\)\(cm\)