A Compound Lens Is Made by Joining the Plane Surfaces

Multiple Choice

A compound lens is made by joining the plane surfaces of two thin plano-convex lenses of different glasses.The radius of curvature of each convex surface is 80 cm.The indices of refraction of the two glasses are 1.50 and 1.60.What is the focal length of the compound lens?

A) 67
B) 69
C) 71
D) 73
E) 75