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osanseviero

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)

  • 5 months ago
  • 5 months ago

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  1. Easyaspi314
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    a) thats a combination of 3 from 5..or 5 C 3

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

    • 5 months ago
  3. osanseviero
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    :/

    • 5 months ago
  4. Hero
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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 months ago
  5. osanseviero
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    Ok, let me check :)

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

    • 5 months ago
  7. Hero
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    You should do more research on permutations.

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

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

    • 5 months ago
  10. osanseviero
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    Thanks :)

    • 5 months ago
  11. Hero
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    yw

    • 5 months ago
  12. osanseviero
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    Yep, this helped me a lot :)

    • 5 months ago
  13. osanseviero
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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?)

    • 5 months ago
  14. Hero
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    They're not the same :)

    • 5 months ago
  15. osanseviero
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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?

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

    • 5 months ago
  17. osanseviero
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    So it is 125 and 10 :)

    • 5 months ago
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