Question

2. An object is 20.0 cm from a converging lens, and the image falls on a screen. When the object is moved 4.00 cm closer to the lens, the screen must be moved 2.70 cm farther away from the lens to register a sharp image. Determine the focal length of the lens.

Answer 1

we have, since the focal length is constant,

1/f = 1/20 + 1/x = 1/16 + 1/(x+2.7)

Just solve for x, and then find f.