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Thread: elevator weight question! urgent help !

  1. September 17th 2009, 12:55 PM #1
    tofustar
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    elevator weight question! urgent help !

    normal distribution... the max elevator weight cannot be greater than 1120kg or 16 people. suppose that the weights of lift users are normally distributed with a mean of 68kg and a standard deviation with 8kg.

    what is the probability that 16 people will exceed the weight limit of 1120kg?

    and also what is the probability that 18 people will not exceed the weight limit?


    p.s thank you for your help
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  2. September 18th 2009, 06:05 AM #2
    mr fantastic
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    Quote Originally Posted by tofustar View Post
    normal distribution... the max elevator weight cannot be greater than 1120kg or 16 people. suppose that the weights of lift users are normally distributed with a mean of 68kg and a standard deviation with 8kg.

    what is the probability that 16 people will exceed the weight limit of 1120kg?

    and also what is the probability that 18 people will not exceed the weight limit?


    p.s thank you for your help
    Weight of 16 people: X ~ Normal \left( \mu = 16 \times 68 = ...., \sigma = 16 \times 8 = ....\right).

    Calculate \Pr( X > 1120).


    Weight of 18 people: Y ~ Normal \left( \mu = 18 \times 68 = ...., \sigma = 18 \times 8 = .... \right).

    Calculate \Pr( Y < 1120).
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  3. September 18th 2009, 08:12 AM #3
    CaptainBlack
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    Quote Originally Posted by mr fantastic View Post
    Weight of 16 people: X ~ Normal \left( \mu = 16 \times 68 = ...., \sigma = 16 \times 8 = ....\right).

    Calculate \Pr( X > 1120).
    \sigma=8 \sqrt{16}

    or \sigma^2= 8^2 \times 16


    Weight of 18 people: Y ~ Normal \left( \mu = 18 \times 68 = ...., \sigma = 18 \times 8 = .... \right).

    Calculate \Pr( Y < 1120).
    same again but with 18 in place of 16.

    CB
    Last edited by CaptainBlack; September 18th 2009 at 08:50 AM.
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  4. September 18th 2009, 09:24 AM #4
    mr fantastic
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    Quote Originally Posted by CaptainBlack View Post
    \sigma=8 \sqrt{16}

    or \sigma^2= 8^2 \times 16




    same again but with 18 in place of 16.

    CB
    Thanks for that catch. Careless of me. (Perhaps my evil nature asserting itself).

    @OP: X = W_1 + W_2 + .... + W_n \Rightarrow Var (X) = n Var (W) \Rightarrow sd (X) = \sqrt{n} sd (W)
    Last edited by mr fantastic; September 18th 2009 at 09:39 AM.
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