Find the lateral magnification produced by the combination of lenses. Shown in figure

If lenses are placed in contact, the focal length of single equivalent length can be given as 
 $\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{1}{10}-\frac{1}{20}=\frac{1}{20}\Rightarrow f=+20cm$
Now using lens formula considering system of lenses as a single lens, we get 
 $$\begin{gathered} \frac{1}{v}-\frac{1}{u}=\frac{1}{f} \therefore \frac{1}{v}--\frac{1}{-10}=\frac{1}{20} \\ \frac{1}{v}=\frac{1}{20}-\frac{1}{10}=\frac{-1}{20}\Rightarrow v=-20cm \end{gathered}$$
Now using magnification  formula, we get 
$m=\frac{-20}{-10}=2$