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s3 estimation - arghhhhhhhh

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AQA GCSE physics P1 unofficial mark scheme 05-05-2016
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    i am stumped!

    an electrical company repairs very large numbers of television sets and wishes to estimate the mean time taken to repair a particular fault. it is known from previous research that the standard deviation of the time taken to repair this fault is 2.5 minutes. the manager wishes to ensure that the probability that the estimate differs from the true mean by less than 30 seconds is 0.95. find how large a sample is required.

    i reckon that p(|e - μ | < 0) < 0.95 where e is the estimate

    is that correct? and what on earth do i do next? thanks so much for the help, i'm self-teaching and feeling very discouraged!
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    i would still really like some help on this!
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    (Original post by manyriverstocross)
    i reckon that p(|e - μ | < 0) < 0.95 where e is the estimate
    I got P(|μ-x̄|<30)=0.95, where μ is the true mean and x̄ is the estimated mean. To calculate this you will need x̄ - was there a previous part to this question?


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