A set of solar batteries is used in a research satellite. The
satellite can run on only one battery, but it runs best if more
than one battery is used. The variance σ2 of lifetimes of these
batteries affects the useful lifetime of the satellite before it
goes dead. If the variance is too small, all the batteries will
tend to die at once. Why? If the variance is too large, the
batteries are simply not dependable. Why? Engineers have...