Answer
$f=2.50~cm$
Work Step by Step
Given
$\frac{1}{f}=\frac{1}{S_{0}}+\frac{1}{S_{i}}$
$f=?, S_{0}=3cm, S_{i}=15cm$
$\frac{1}{f}=\frac{1}{3}+\frac{1}{15}$
Therefore, $f=2.50~cm$
How to use the calculator:
$3,*,x^{-1},+,15,*,x^{-1},=,x^{-1},=.$
