A man who can produce sharp images on his retina only of objects that lie from 100 cmcm to 250 cmcm from his eyes needs bifocal lenses.

Part A:

Determine the desired focal length of the upper half of the glasses used to see distant objects.

Express your answer with the appropriate units.

Part B:

Determine the focal length of the lower half used to read a paper held 25 cmcm from his eyes.

Express your answer with the appropriate units.