Answer
See below.
Work Step by Step
$\frac{1}{f}=(n-1)[\frac{1}{R_1}+\frac{1}{R_1}]$. Thus after taking the reciprocal of both sides: $f=\frac{1}{(n-1)[\frac{1}{R_1}+\frac{1}{R_1}]}$
hence in this case $f=\frac{1}{(1.5-1)[\frac{1}{0.1}+\frac{1}{0.2}]}=\frac{1}{(0.5)[10+5]}=\frac{1}{7.5}$
