Answer:
f = 5 cm
Explanation:
using the thin lens equation, given as follows:
[tex]\frac{1}{f} = \frac{1}{d_{o}}+\frac{1}{d_{i}}\\[/tex]
where,
f = focal length = ?
do = the distance of object from lens = 20 cm
di = the distance of image from lens = 6.6667 cm
Therefore,
[tex]\frac{1}{f} = \frac{1}{20\ cm}+\frac{1}{6.6667\ cm}\\\\\frac{1}{f} = 0.199999\ cm^{-1}\\\\f = \frac{1}{0.199999\ cm^{-1}}\\\\[/tex]
f = 5 cm
