Elmer has a collection of 300 fossils. Of these, 21%, percent are fossilized snail shells.
How many fossilized snail shells does Elmer have?
Answers
A greatly simplified view of the human eye is shown below. The pupil is a little hole which allows light to pass into the eye. Behind the pupil lies the eye's lens. Muscles in the eye control the size of the pupil and the shape of the lens, thereby adjusting the amount of light that enters they eye and the focus of the lens. The retina is a sensitive layer of nerves at the back of the eyeball; incident light upon the retina is translated into a coherent image by the brain.

Many people do not have perfect vision; that is, a lot of people have eyes whose lenses do not focus light properly on the retina. Two well-known vision problems correctible via eyeglasses are nearsightedness (picture (a) above) and farsightedness (picture (c)). Nearsightedness focusses rays of light in front of the retina, while farsightedness focusses rays behind the retina. A diverging lens can correct nearsightedness by bending incoming light rays outwards, so that the eye's lens (which usually bends incoming rays too much) focusses the light closer to the retina (picture (b)). A converging lens similarly corrects farsightedness