Problem 5.31

A point source is located at a distance of $20 \mathrm{~cm}$ from the front surface of a symmetrical glass biconvex lens. The lens is $5.0 \mathrm{~cm}$ thick and the curvature radius of its surfaces is $5.0 \mathrm{~cm} .$ How far beyond the rear surface of this lens is the image of the source formed?