The figure shows that the radius of curvatures of the two lens surfaces are each 40 cm.
The focal length is half the radius of curvature.
R = 40 cm
FL = R/2 = 20 cm.
Hope this helps,
Actually the correct answer is 40 cm. Take those two arrows and slide them along the lens, you’ll see that they both touch at the x-axis.
Source(s): I just did it on MasteringPhysics…