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Math Help - permutations and combinations

  1. #1
    Junior Member
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    Jun 2009
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    permutations and combinations

    How many numbers greater than 4000 can be formed from the figures 3, 5, 7, 8, 9 (repetitions not allowed)

    my working out was that

    _ _ _ _

    for the first place, there are 4 possibilities, for the second digit, there are still 4, for the third there are 3 and for the fourth there are 2, so i multiplied 4x4x3x2
    but the answer is 216

    i also went for a different approach
    where there are three digits and 5 numbers to fill it
    SO it becomes 5P3 where 5!/5!-3! = 60 and i multipled that by 4 cause there are 4 numbers possible for the first digit. but that gave me 240
    eurghz i dont get it ! ><"" sorryy if anyone could help me i would realli realli appreciate it !!
    thankyou ! =D
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  2. #2
    Super Member
    Joined
    May 2009
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    Where does it say that the numbers can only be 4 digits long?

    If we look at numbers that are only 4 digits long, then yes, the number of possibilities would be
    4 x 4 x 3 x 2 = 96.

    But then we also have to look at 5 digit numbers, and the number of possibilities for those would be
    5 x 4 x 3 x 2 x 1 = 120.

    Add them together and you get 216.


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  3. #3
    Junior Member
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    omgsh thank you so much !
    iloveyou =D
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