A convex lens is in contact with concave lens. The magnitude of the ratio of their focal length is $\frac{3}{2}$. Their equivalent focal length is 30 cm. What are their individual focal lengths?

Let focal length of convex lens is , + f then focal length of concave lens would be -$\frac{3f}{2}$, 

$\frac{1}{30}=\frac{1}{f}-\frac{2}{3f}=\frac{1}{3f}$

$\text{ From the given condition, }\therefore f=10\mathrm{cm}$

Therefore, focal length of convex lens = +10cm and that of concave lens = -15cm

Note Since the combination is behaving like a convex lens (as F is positive)

Hence, power of convex lens is more or its focal length is less.