Answer
$f=10.7~cm$
Work Step by Step
Given
$\frac{1}{f}=\frac{1}{S_{0}}+\frac{1}{S_{i}}$
$f=?, S_{0}=16.5cm, S_{i}=30.5cm$
$\frac{1}{f}=\frac{1}{16.5}+\frac{1}{30.5}$
Therefore, $f=10.7~cm$
How to use the calculator:
$16.5,*,x^{-1},+,30.5,*,x^{-1},=,x^{-1},=.$
