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Probability Question

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  • 02-11-2016 5:33pm
    #1
    Jan_de_Bakker
    Closed Accounts Posts: 1,403 ✭✭✭


    Hi,
    A bowl contains two Snickers bars, two Mars bars, and one packet of sunflower seeds. I pick two items out of the bowl at random with replacement. Find the chance that
    a) I get the sunflower seeds ... snip

    OK so this part a should be the easiest part, I got the other more complex parts right so I don't understand why the answer to this is 9/25 (0.36)

    I get 2/5 (0.4)

    My reasoning

    5 items in total 1 pack of sunflower seeds.

    1st run 1/5

    2nd run (with replacement) 1/5

    its or , so 1/5 + 1/5

    ?

    Is it because Im not including the probability of getting 2 in a row ?


Comments

  • Options
    #2
    MarinersBlues
    Registered Users Posts: 1,290 ✭✭✭MarinersBlues


    I calculate the chance of not getting the sunflower seeds on either attempt and subtract from 1.

    (4/5)*(4/5)=16/25

    1-(16/25)=9/25


  • Options
    #3
    MarinersBlues
    Registered Users Posts: 1,290 ✭✭✭MarinersBlues


    Alternatively

    Getting them 1st time only
    (1/5)*(4/5)=4/25
    Getting them 2nd time only
    (4/5)*(1/5)=4/25
    Getting them both time
    (1/5)*(1/5)=1/25

    and sum = 9/25


  • Options
    #4
    Deise Vu
    Registered Users Posts: 1,944 ✭✭✭Deise Vu


    The answer is 100%. Who can't tell the difference between a packet of Sunflower Seeds and a Mars / Snickers?


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    #5
    Jan_de_Bakker
    Closed Accounts Posts: 1,403 ✭✭✭Jan_de_Bakker


    Thanks, of course so easy when I see it that way ..

    I had gotten 9/25 by thinking

    25 total outcomes

    and these are the 9 im interested in ..

    sf/sf
    sf/mars1
    sf/mars2
    mars1/sf
    mars2/sf
    sf/snick1
    sf/snick2
    snick1/sf
    snick2/sf

    But that way is ****e ... thanks


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    #6
    Jan_de_Bakker
    Closed Accounts Posts: 1,403 ✭✭✭Jan_de_Bakker


    Actually, if I do the same without replacement ..

    its 8/20

    1 - P(not getting sflower)

    1 - [4/5 * 3/4]


    But if I do :

    getting 1st time only 1/5 * 3/4 = 3/20
    getting 2nd time 4/5 * 1/4 = 4/20
    both times (impossible here)


    sum 7/20 :confused: ?


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  • Options
    #7
    spock.
    Closed Accounts Posts: 8,502 ✭✭✭spock.


    Jan_de_Bakker wrote: »
    Actually, if I do the same without replacement ..

    its 8/20

    1 - P(not getting sflower)

    1 - [4/5 * 3/4]


    But if I do :

    getting 1st time only 1/5 * 3/4 = 3/20
    getting 2nd time 4/5 * 1/4 = 4/20
    both times (impossible here)


    sum 7/20 :confused: ?

    Getting it the first time only is actually 1/5 * 4/4 = 1/5
    If you got it on the first attempt you can't get it again!

    This leads to 1/5 + 1/5 = 2/5 = 8/20


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