Question

the life time of a certain kind of mean life of 400 hours and standard deviation of 45 hours. assuming the distribution of life time to be normal, find : 1. the percentage of batteries with a life time of atleast 470 hours 2. the proportion of batteries with a lifetime between 385 and 415 hours 3. the minimum life of the best 5% of the batteries

Answer 1

Z = (score-mean)/SD

1. Z = (470-400)/45 = ?

2. Use same equation for both values.

3. Find Z score in table and work backwards.

Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion/probability related to the Z score(s).