Answer:
According to the question –
Focal length of the convex lens is f1=30cm
Focal length of the concave lens is f2= -20cm
Focal length of the system of lenses is denoted by f
The equivalent focal length of the given system of lenses is given as:
$\frac{1}{f}=\frac{1}{f_{1}}+\frac{1}{f_{2}}$
$\frac{1}{f}=\frac{1}{30}-\frac{1}{20}=\frac{2-3}{60}=-\frac{1}{60}$
Therefore f=-60cm
As a result, the focal length of the lens combination is 60cm. In addition, the negative sign indicates that the lens system in use is a diverging lens.