The lifetime of a certain brand of electric light bulb is known to have a standard deviation of

55

hours. Suppose that a random sample of

60

bulbs of this brand has a mean lifetime of

499

hours. Find a

95%

confidence interval for the true mean lifetime of all light bulbs of this brand. Then give its lower limit and upper limit.

Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place. (If necessary, consult a list of formulas.