What is the density of free electrons in the metal? ?

Plss anybody know how to do this? ? i need the answers from a through d

With the simplified model of the eye, what corrective lens (specified by focal length as measured in air) would be needed to enable a person underwater to focus an infinitely distant object? (be careful-the focal length of a lens underwater is not the same as in air! assume that the corrective lens has a refractive index of 1.62 and that the lens is used in eyeglasses, not goggles, so there is water on both sides of the lens. assume that the eyeglasses are 1.90

Write down two additional questions about earth's magnetic field that will make your presentation more informative.