Probablity

The life time of a certain kind of batteries has a mean life of 400 hours and standard deviation as 45 hours. Assuming the distribution of life to be normal, find i) the percentage of batteries with a life time of atleast 470 hours. Ii) Out of 10000 batteries how many batteries do you expect to have life between 385 and 415 hours.

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  1. This page will replace any tables or charts in the back of texts or handouts you might have been given

    http://davidmlane.com/hyperstat/z_table.html

    enter:
    mean -- 400
    SD -- 45
    click on "above" and enter 470
    prob is .0599 ---> 5.99%

    ii) click on "between" and enter the values
    multiply the prob by 10000

    The beauty of this page is that you don't have to find z-scores first.
    If your course requires you to work with z-scores, then find those scores for all your numbers, and set the mean to 0 and the SD to 1, then enter the data as z-scores.
    You will get the same results.

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