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

-15, 10

$\frac{f_1}{f_2}=\frac{2}{3}$                      (1)

$\frac{1}{f_1}-\frac{1}{f_2}=\frac{1}{30}$           (2)

Solving equations (1) and (2),

⇒ f₂ = 15 cm (concave lens)

f₁ = 10 cm (convex lens)

∴ (d)