Answer
$60.7cm$
Work Step by Step
We can find the required focal length as follows:
$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$
We plug in the known values to obtain:
$\frac{1}{f}=\frac{1}{25.0cm}+\frac{1}{-42.5cm}$
$\frac{1}{f}=\frac{17.5}{1062.5}cm$
$f=\frac{1062.5}{17.5}cm$
$f=60.7cm$