Suppose that in operating battery-powered electrical equipment, it is less expensive to replace all batteries at fixed intervals than to replace each battery individually when it breaks down, provided the standard deviation of the lifetime is less than a certain limit, say, less than 5 hours, Set up and apply a suitable test, using a sample of 28 values of lifetimes with standard deviation s = 3.5 hours and assuming normality; choose a = 5%?