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Math Help - can some one help me please

  1. #1
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    can some one help me please

    • The mean lifetime of a particular type of electric light bulb is 1400 hours, with a standard deviation of 300 hours.

    What is the probability that this sample will yield a mean of less than 1390 hours?
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  2. #2
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    correct its 1390-1400/300 = .033 then on the z table =.5120
    p z >.033=.5120
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  3. #3
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    Quote Originally Posted by crashuk View Post
    correct its 1390-1400/300 = .033 then on the z table =.5120
    p z >.033=.5120
    1. z = -0.033, NOT 0.033.

    2. You require Pr(Z < -0.033), NOT Pr(Z > 0.033) and not, for that matter, Pr(Z > -0.033).

    3. Although I can figure out most of what you're saying in answer to your own question, if you wrote that way on a homework or test paper in a course of mine, you'd get points deducted from your grade. What you mean (I think) is:

    "z = 1390-1400/300 = .033. Then from the z table, Pr(z < 0.033) =.5120"

    (I have no idea where "p z >.033=.5120" has come from")

    Sure, it takes a few more words to say that, but words matter. To communicate mathematics, you have to say exactly what you mean - no more, no less.
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  4. #4
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    • The mean lifetime of a particular type of electric light bulb is 1400 hours, with a standard deviation of 300 hours.
    What is the probability that this sample will yield a mean of less than 1390 hours?
    so 1390-1400/300 = -0.033 = 51.20% probability that the sample would yield 1390 hours
    which would = p(z < 0.033)=.5120
    so if it a negative value then i have to look at negative values on the z table. which then would mean .4840
    p(z < 0.033)=.4840
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  5. #5
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    Quote Originally Posted by crashuk View Post
    • The mean lifetime of a particular type of electric light bulb is 1400 hours, with a standard deviation of 300 hours.
    What is the probability that this sample will yield a mean of less than 1390 hours?
    so 1390-1400/300 = -0.033 = 51.20% probability that the sample would yield 1390 hours
    which would = p(z < 0.033)=.5120
    so if it a negative value then i have to look at negative values on the z table. which then would mean .4840
    p(z < 0.033)=.4840
    I get 0.4867.
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  7. #7
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    Well, you can only be as accurate as the available tool - not your fault.
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  8. #8
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    Quote Originally Posted by mr fantastic View Post
    Well, you can only be as accurate as the available tool - not your fault.
    thanks mr fantastic youve been a help, sometimes getting the answer helps you understand.
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