3) Find out the Focal length for the following lenses (in all cases, the lens is made of a glass with index of 1.5):

Question

a) Bi-convex lens with R, v 30cm.V R, V 20cm,
b) Planar-concave lens with radius of Rv 50cm,
c) Meniscus convex lens with R v 30cm, R, V 20cm.

I need this answer in like 30 minutes if possible, thank u so much!