A manufacturer of cell phone batteries knows that its products lasts an average (mean) of 40months with a standard deviation of 8 months.
a) if they guarentee them for 2.5years(30months), what % will they have to replace?
b) if they do not wish to replace more than 3% of battereis, what should be the length of the guarantee?

Question

A manufacturer of cell phone batteries knows that its products lasts an average (mean) of 40months with a standard deviation of 8 months.
a) if they guarentee them for 2.5years(30months), what % will they have to replace?
b) if they do not wish to replace more than 3% of battereis, what should be the length of the guarantee?

mathmate · Answer

Here $\mu=40$ months, $\sigma=8$.
a) calculate Z for x=30 months: $Z=\frac{x-\mu}{\sigma}$ and then look up the normal distribution table for the probability of x falling below Z.  Look for the one-tail table.
b) is the reverse process of a).  You are given a probability.  Look for the corresponding Z from the table and calculate the number of months x.

anonymous · Answer

thank you sooo much! this site and you guys are a great help to my statistics class!!!

mathmate · Answer

You're welcome!  :)