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Math Help - permutation and combination

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    permutation and combination

    4 alphabets are selected from the alphabets in the word LEMBAGA. Find the number of selection of 4 different alphabets which can be made so that it
    a. contains only 1 alphabet A
    b. contains 2 alphabets A

    I always get confuse with this type of question, so...
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  2. #2
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    Re: permutation and combination

    Quote Originally Posted by Trefoil2727 View Post
    4 alphabets are selected from the alphabets in the word LEMBAGA. Find the number of selection of 4 different alphabets which can be made so that it
    a. contains only 1 alphabet A
    b. contains 2 alphabets A
    The answers are \\a)~\binom{5}{3}\\b)~\binom{5}{2}

    Now if you ever want more help here, you must explain why those are correct.
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    Re: permutation and combination

    Quote Originally Posted by Plato View Post
    The answers are \\a)~\binom{5}{3}\\b)~\binom{5}{2}

    Now if you ever want more help here, you must explain why those are correct.
    em, I thought
    a) 4 alphabets are selected from 7 alphabets, of the 7, 2 are alphabets A. So, 7-2=5, then I thought I have to use 5C3 x 2C1( because there are 2 alphabets A)
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  4. #4
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    Re: permutation and combination

    Quote Originally Posted by Trefoil2727 View Post
    em, I thought
    a) 4 alphabets are selected from 7 alphabets, of the 7, 2 are alphabets A. So, 7-2=5, then I thought I have to use 5C3 x 2C1( because there are 2 alphabets A)
    But the two A's are identical. We cannot tell one from the other. So there is only one way to select an A.
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