a convex lens of power 3.5D is placed in contact with a concave lens of power -1.5D. Find the effective focal length of the combination.

a concave lens of focal length 20cm is placed in contact with a convex lens of focal length 40cm. Find the power of the combination.

Two lenses of powers +5D and - 3D are in contact: Find the focal length of the combination.

Two lenses of powers +5D and 3D are in contact.Find the focal length of combination.

Two lenses of powers, 15D and +5D are in contact with each other forming a combination lens. What is the focal length of the combination?