Hello! Posting this here cause my girlfriend is having trouble with a math problem and I can't really figure out what to do either.
A manufacturer of batteries tested the lifespan of a sample of batteries. Mean life was found to be 485 days with standard deviation of 26 days. These batteries are used in beacons that are switched off for 10 hours a day. After how many days should the batteries be changed to ensure that less than 2% of the beacons experience battery failure.
Now I tried attacking this from several angles but basically had to resort to guesstimating values of the Z against a plot, and they expect her to do it without a graphical calculator. She does have a normal distribution Z table for reference though.
Thanks for your help!
A manufacturer of batteries tested the lifespan of a sample of batteries. Mean life was found to be 485 days with standard deviation of 26 days. These batteries are used in beacons that are switched off for 10 hours a day. After how many days should the batteries be changed to ensure that less than 2% of the beacons experience battery failure.
Now I tried attacking this from several angles but basically had to resort to guesstimating values of the Z against a plot, and they expect her to do it without a graphical calculator. She does have a normal distribution Z table for reference though.
Thanks for your help!