Suppose the life of a particular brand of calculator battery is approximately normally distributed with a mean of

80

hours and a standard deviation of

9

hours. Complete parts a through c.

c. If the manufacturer of the battery is able to reduce the standard deviation of battery life from

9

to

7

hours, what would be the probability that

4

batteries randomly sampled from the population will have a sample mean life of between

70

and

90

hours?