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Project Maths: Probability teaser. (help)

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  • 21-05-2012 11:37am
    #1
    Stalin and rugby
    Closed Accounts Posts: 243 ✭✭


    Sup tweeps been on this for ages so any help is appreciated. Q 7. pg 127 in the blue project maths book

    "In an examination a candidate is required to select any 7 questions from ten.

    (i) In how many ways can this be done?
    (ii) How many of the selections contain the first and second questions?
    Now calculate the probability that the candidate selects

    (iii) both the first and second questions.
    (iv) at least one of the first to questions.

    Answers 120 , 56 , 7/15 , 14,15 ,
    Tagged:


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    #2
    BeanbagBallbag
    Registered Users Posts: 181 ✭✭BeanbagBallbag


    How many selections don't contain the first two?
    How many contain just 1st without 2nd?
    How many contain just 2nd without 1st?

    The number of combinations left after you take out those is how many contain both 1st + 2nd. Try it that way.


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    #3
    Local_Chap
    Registered Users Posts: 570 ✭✭✭Local_Chap


    Is the answer to (i) 720 and (ii) 336


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    #4
    FaoiSin
    Registered Users Posts: 789 ✭✭✭FaoiSin


    Think that's it for part (ii)


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    #5
    reznov
    Registered Users Posts: 921 ✭✭✭reznov


    Automatically, we choose two questions so out of a possible 10 we have 8 remaining.
    Since we need to answer 7 questions, out of the remaining 8 choose any 5. (Guy in question is a newb because he couldn't answer all 10, but that's irrelevant)
    And we have the answer of 56.


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    #6
    FaoiSin
    Registered Users Posts: 789 ✭✭✭FaoiSin


    Part (iii) and (iv) ... I've gotten rusty on these...


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    #7
    Stalin and rugby
    Closed Accounts Posts: 243 ✭✭Stalin and rugby


    YeaaahFaoiSin wrote: »
    Part (iii) and (iv) ... I've gotten rusty on these...

    By the looks of things you have not :D


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    #8
    FaoiSin
    Registered Users Posts: 789 ✭✭✭FaoiSin


    Despise this kind of Math


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    #9
    ConorSTF
    Registered Users Posts: 119 ✭✭ConorSTF


    The answer to the first part of the question is 120.

    There are 10 options, you choose 7 = 10C7 = 120.

    That's all my brain is up for at the moment, although I am off to study Probability now. :)


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