A Double-Convex Lens Made of Glass Having a Refractive Index

Multiple Choice

A double-convex lens made of glass having a refractive index of 1.50 has a radius of magnitude 40 cm on the front side and one of magnitude 30 cm on the back side. What is the focal length of this
Lens?

A) 11 cm
B) 80 cm
C) 34 cm
D) -11 cm
E) -240 cm