SOLUTION: The lifetime in hours of a brand of light bulb is normally distributed with a mean of 221 hours and a standard deviation of 7 hours. a) What is the probability that a light bulb

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Question 1180992: The lifetime in hours of a brand of light bulb is normally distributed with a mean of 221 hours and a standard deviation of 7 hours.
a) What is the probability that a light bulb lasts between 215 and 234 hours?
b) What is the lifetime of a light bulb that is at the 85th percentile?
Answer by Theo(13342) About Me  (Show Source):
You can put this solution on YOUR website!
mean is 221 hours.
standard deviation is 7 hours.

a) What is the probability that a light bulb lasts between 215 and 234 hours?

the probability is .7727, using the calculator that can be found at https://davidmlane.com/hyperstat/z_table.html

here's a display of the use of that calculator to solve this problem.



b) What is the lifetime of a light bulb that is at the 85th percentile?

the lifetime of a bulb at the 85th percentile is 228.255, using the same calculator as above, only using the value from an area option rather than the area from a value option as was done solving the previous question.

here's a display of the use of that calculator to solve this problem.



the 85th percentile means the lifetime of the bulb is greater than 85% of all the bulbs.