1. Subscribervenda
    Dave
    S.Yorks.England
    Joined
    18 Apr '10
    Moves
    83468
    05 Apr '21 14:49
    I think I have got the necessary formula in a spreadsheet now so if you could indulge me (just one more time hopefully!! ) ,I have tried some random numbers in it:-
    If there are 30 people in the class and the set is a b c d e f what is the probability that 4 pupils read set a b e and f?
    My spreadsheet says 9%
    Is this right?
  2. R
    Standard memberRemoved
    Joined
    10 Dec '06
    Moves
    8528
    05 Apr '21 15:37
    @venda said
    I think I have got the necessary formula in a spreadsheet now so if you could indulge me (just one more time hopefully!! ) ,I have tried some random numbers in it:-
    If there are 30 people in the class and the set is a b c d e f what is the probability that 4 pupils read set a b e and f?
    My spreadsheet says 9%
    Is this right?
    Ok, so now there is 6 books to choose from, and they are instructed to choose 4 of them to read.

    so a,b,e,f is 1 of C(6,4) = 15

    With 30 students having exactly 4 of them read the desired subset there are

    C( 30,4 ) ways to select students to read that set and the remaining 26 students have 14 subsets to choose from

    There are C( 30,4)*14^( 30-4 ) ways to do this.

    The total number of ways for 30 students to select any subset to read is:

    15^30

    Thus, the probability of exactly 4 of them reading the subset a,b,e,f is given by ( assuming all subsets are equally likely to be chosen ):

    P( exactly 4 read books a,b,e,f ) = C( 30,4)*14^( 30-4 )/15^30 ≈ 9%

    Good Job Venda!
  3. Subscribervenda
    Dave
    S.Yorks.England
    Joined
    18 Apr '10
    Moves
    83468
    06 Apr '21 07:52
    @joe-shmo said
    Ok, so now there is 6 books to choose from, and they are instructed to choose 4 of them to read.

    so a,b,e,f is 1 of C(6,4) = 15

    With 30 students having exactly 4 of them read the desired subset there are

    C( 30,4 ) ways to select students to read that set and the remaining 26 students have 14 subsets to choose from

    There are C( 30,4)*14^( 30-4 ) ways to ...[text shortened]... ):

    P( exactly 4 read books a,b,e,f ) = C( 30,4)*14^( 30-4 )/15^30 ≈ 9%

    Good Job Venda!
    Thanks Joe.
    Now I have the method (I used bigdoggs formula)I can now tidy up my spreadsheet a bit so for any similar type problems I can just plug in the numbers and it'll give me the answer.

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