Answer:
A certain company makes 12-volt car batteries. After many years of product testing, the company knows that the average life of a battery is normally distributed, with a mean of 50 months and a standard deviation of 9 months. If the company does not want to make refunds for more than 10% of its batteries under the full-refund guarantee policy, for how long should the company guarantee the batteries?
The company should guarantee the batteries for 38 months.
Step-by-step explanation:
Using standard normal table,
P(Z < z) = 10%
=(Z < z) = 0.10
= P(Z <- 1.28 ) = 0.10
z = -1.28
Using z-score formula
x = zσ + μ
x = -1.28 *9+50
x = 38
Therefore, the company should guarantee the batteries for 38 months.
