Probability or Algebra ?

Discussion in 'Computer Support' started by Angela Singh, Sep 13, 2006.

  1. Angela Singh

    Angela Singh Guest

    Hello,

    I need two lots of help - firstly is this Algebra or Probability,
    and secondly - what is the answer ?

    I made a start (below) but have reached my limit :(

    --------------------------------------------------------------------------------------------------------------
    Two pupils were to be chosen at random from a school register to take
    part in a competition. The probability that both would be boys was
    one- third. Before the choice could be made however, a decision was
    taken to include pupils from the register of another school in the
    ballot for the two places. This other chool had a register of 1,000
    pupils, and the chance that the two selected pupils would both be boys
    was reduced to one-thirteenth.

    How many pupils are on the register of the first school ?
    -------------------------------------------------------------------------------------------------------------

    So far i have worked out that if the chance of two boys from school A
    is one third, then the chance of the first picked being a boy is
    1/sqrt*3 which is approx 57.7%
    Therefore school A's breakdown is 57.7% male and 42.2% female

    Population od school B = 1000

    The chance of 2 boys from (school A + 1000) = 1/13

    therefore the chance of 2 boys = (A + 1000) = 27.7%

    therefore the breakdown of (A+1000) = 27.7% male and 72.3% female


    Many thanks,

    Angela
     
    Angela Singh, Sep 13, 2006
    #1
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  2. Angela Singh

    richard Guest

    First, read what the question wants as an answer. That being, the population
    of school A.

    What number gives 2 as 1/3 of that number?
    6.

    The rest of the problem is there only to throw you off.

    Like in the age old riddle, "There once was a man who was going to Tipperary
    by train. At the train station he meets a woman with 7 kids who was going to
    London. Each of the kids had 7 goats.............How many were going to
    Tipperary?"

    You have two coins totalling 55 cents. One is not a half dollar coin. What
    is the other?
     
    richard, Sep 13, 2006
    #2
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  3. Angela Singh

    Angela Singh Guest

    Thank you Richard, but - If school A has 6 pupils, then
    the chance of the first selected being a boy is 3 out of 6 (= 1/2)
    and then there are only 5 pupils left in the population because one
    has already been selected and removed.

    The probability that the first two selected are boys = 1/3

    therefore, chance of first two being being boys is (1/2) * (X) = 1/3

    therefore the chance of the second selected being a boy (X) = 2/3

    And what about the chance of the first two selected pupils being boys
    from school A + school B (6+1000) being one-thirteenth ?

    I have to submit this answer by tomorrow but i want to understand it
    first.

    Thanks,
    Angela







    --------------------------------------------------------------------------------------------------------------
    Two pupils were to be chosen at random from a school register to take
    part in a competition. The probability that both would be boys was
    one- third. Before the choice could be made however, a decision was
    taken to include pupils from the register of another school in the
    ballot for the two places. This other chool had a register of 1,000
    pupils, and the chance that the two selected pupils would both be boys
    was reduced to one-thirteenth.

    How many pupils are on the register of the first school ?
     
    Angela Singh, Sep 14, 2006
    #3
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