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osanseviero

  • 2 years ago

If I have 5 letters (A,B,C,D,E), and I want to form groups of three letters. a) How many can I form? (AAA, AAB, AAC, etc) b) How many without repeating? (ABC, ABD, etc)

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  1. anonymous
    • 2 years ago
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    a) thats a combination of 3 from 5..or 5 C 3

  2. osanseviero
    • 2 years ago
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    So 125 in the first one, and 20 in the second one. Why is it n(n-1)?

  3. osanseviero
    • 2 years ago
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    :/

  4. Hero
    • 2 years ago
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    If I'm not mistaken the first one is a permutation since elements are allowed to repeat. Which means the total number of three letter formations should be \(n^r=5^3 = 125\) The second question is a combination since elements are not allowed to repeat. So I imagine that it is \(5C3\)

  5. osanseviero
    • 2 years ago
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    Ok, let me check :)

  6. osanseviero
    • 2 years ago
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    I searched an in a permutation elements cant be repeated :/

  7. Hero
    • 2 years ago
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    You should do more research on permutations.

  8. osanseviero
    • 2 years ago
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    So the basic formulas are \[n ^{r}\] if they can be repeated and NCr, when they cant be repeated?

  9. Hero
    • 2 years ago
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    This resource might help clear things up a bit more: http://www.mathsisfun.com/combinatorics/combinations-permutations.html

  10. osanseviero
    • 2 years ago
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    Thanks :)

  11. Hero
    • 2 years ago
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    yw

  12. osanseviero
    • 2 years ago
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    Yep, this helped me a lot :)

  13. osanseviero
    • 2 years ago
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    I am seeing that the formula of the combinations without repetition and permutation without repetition are the same. How can I know if this if a permutation or a combination? (the order matters here?)

  14. Hero
    • 2 years ago
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    They're not the same :)

  15. osanseviero
    • 2 years ago
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    oops, not the same, I reduce by r one more time. But how can I know in this case if it is permutation or combination?

  16. osanseviero
    • 2 years ago
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    (from the question?, how can I know if order matters?

  17. osanseviero
    • 2 years ago
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    So it is 125 and 10 :)

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