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someone handy with probablity help out please

  • 22-10-2009 09:48PM
    #1
    imported_guy
    Closed Accounts Posts: 1,141 ✭✭✭


    hey guys, 2nd yr mechanical engineering ive been falling back in class a bit and need some help :<

    An amplifier consists of 2 independent stages, A and B, connected in series, The output of A is the input to B. The amplifier works if both stages A and B work. If the probablity of A working is 0.72, and the probability of the amplifier working is 0.61 then find the probability that stage B works given that stage A works

    and

    The lengths of certain machined components are normally distributed with a mean of 1.107m and standard deviation of 0.2m. Find the values that constitute the central 90% of the lengths

    thank you very very much :)


Comments

  • #2
    LeixlipRed
    Closed Accounts Posts: 6,081 ✭✭✭LeixlipRed


    The first part is condintional probability. If some event happens how does that effect another event. You should have a formula in your notes? The second question deals with confidence intervals. Again, you should have a formula in your notes. Come back if you have trouble.


  • #3
    imported_guy
    Closed Accounts Posts: 1,141 ✭✭✭imported_guy


    LeixlipRed wrote: »
    The first part is condintional probability. If some event happens how does that effect another event. You should have a formula in your notes? The second question deals with confidence intervals. Again, you should have a formula in your notes. Come back if you have trouble.

    thanks alot looking into it now :), edit

    would the 1st one be like a glorified deck of cards problem? or rolling 2 dice getting a certain number over 6?


  • #4
    LeixlipRed
    Closed Accounts Posts: 6,081 ✭✭✭LeixlipRed


    Actually the first one is easier than that. A and B are independent.


  • #5
    BQQ
    Registered Users, Registered Users 2 Posts: 2,192 ✭✭✭BQQ


    First one seems pretty simple.
    If B works 100% of the time the amplifier will work 0.72


  • #6
    imported_guy
    Closed Accounts Posts: 1,141 ✭✭✭imported_guy


    i got the first one and i think i figured out the 2nd

    z = (x-xbar)/sigma

    i think central 90% starts from .2 x .9 = .18 on either side of 1.107

    so i need to find the z score between 1.307 and .097

    am i doing it the right way?


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