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Math Help - combination's of letters

  1. #1
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    Wink combination's of letters

    say you have to find how many 4 letter combination's there are of the word "outlook"

    how do you do that?
    If it where a word with no repeated letters it wouldnt be bad ie 7C4 (Pretty sure) = 7!/(4!3!) = 35

    but what is this, maybe it's 6c4 = 6!/(2!2!) = 180? I know that is wrong the answer is 15.

    Thanks.
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  2. #2
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    Quote Originally Posted by brentwoodbc View Post

    maybe it's 6c4 = 6!/(2!2!) = 180? I know that is wrong the answer is 15.
    Hi, maybe you are right, 6c4 =15
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  3. #3
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    I made a mistake it should have been 6!/2!4! = 15

    but when I do it like that for the others it doesnt work, and it wouldnt work if there where two different letters that where repeated.



    osborne

    7 letter's

    so

    7!/ (4!(7-4)!) = 35 {thats with no repeated letters} right answer is 25

    ?
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  4. #4
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    Quote Originally Posted by brentwoodbc View Post
    "outlook"
    Assume no "o" is used= {4 \choose 4}=1
    Assume 1 "o" is used= {4 \choose 3}=4
    Assume 2 "o" is used= {4 \choose 2}=6
    Assume 3 "o" is used= {4 \choose 1}=4

    This will consider all cases and total =15
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  5. #5
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    Quote Originally Posted by brentwoodbc View Post
    "osborne"
    Assume no "o" is used= {5 \choose 4}=5
    Assume 1 "o" is used= {5 \choose 3}=10
    Assume 2 "o" is used= {5 \choose 2}=10

    All cases= 25
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