Answer
$f=20.0cm$
Work Step by Step
$d_{o1}=60.0cm$
$d_{o2}=40.0cm$
$d_{i2}-d_{i1}=10.0cm$
$d_{i}=\frac{fd_o}{d_o-f}$
$d_{i1}=\frac{fd_{o1}}{d_{o1}-f}=\frac{f(60.0cm)}{(60.0cm)-f}$
$d_{i2}=\frac{fd_{o2}}{d_{o2}-f}=\frac{f(40.0cm)}{(40.0cm)-f}$
$\frac{f(40.0cm)}{(40.0cm)-f}-\frac{f(60.0cm)}{(60.0cm)-f}=10.0cm$
$f=20.0cm$
