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Math Help - Find the mean, variance, and standard deviation

  1. #1
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    Find the mean, variance, and standard deviation

    Q.7
    In an assembly line production of industrial robots, gearbox assemblies can be installed in
    two minute each if holes have been properly drilled in the boxes and in nice minutes if
    the holes must be be redrilled. Twenty gearboxes are in stock, 2 with improperly drilled
    holes. Five gearboxes must be selected from the 20 that are available for installation in the next five robots.

    i) Find the probability that all 5 gearbox will fit properly. (2 marks)
    ii) Find the mean, variance, and standard deviation of the time to install these 5 gearbox

    I have no problem with Question 7(i), but i dont know how to solve 7(ii),
    the following is my solution but somehow it seems to be incorrect, someone please help me.

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  2. #2
    MHF Contributor

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    Quote Originally Posted by extrem3 View Post
    Q.7
    In an assembly line production of industrial robots, gearbox assemblies can be installed in
    two minute each if holes have been properly drilled in the boxes and in nice minutes if
    the holes must be be redrilled. Twenty gearboxes are in stock, 2 with improperly drilled
    holes. Five gearboxes must be selected from the 20 that are available for installation in the next five robots.

    i) Find the probability that all 5 gearbox will fit properly. (2 marks)
    ii) Find the mean, variance, and standard deviation of the time to install these 5 gearbox

    I have no problem with Question 7(i), but i dont know how to solve 7(ii),
    the following is my solution but somehow it seems to be incorrect, someone please help me.

    Well, that's not the mean of the time since you haven't use the fact that the values are 2 minutes and 9 minutes.
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  3. #3
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    Is this correct now?

    Last edited by extrem3; August 27th 2009 at 09:27 AM.
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