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Homework Help: Arrangement of items after removing some of the items

  1. Jul 9, 2014 #1
    1. The problem statement, all variables and given/known data
    A bag contains 4 red cubic blocks, 3 yellow cubic blocks, . Five cubic blocks are removed randomly from the bag and arranged in line . How many arrangements are possible?

    the ans is 8 .

    my working is 7P5/(4! X 3!) =17.5 .

    my ans is rather weird.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jul 9, 2014 #2


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    Are we to assume that blocks of the same color are indistinguishable?
  4. Jul 9, 2014 #3

    Ray Vickson

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    The number of arrangements cannot possibly be 17.5; it cannot be fractional.

    Instead of just writing down some formulas, you need to explain the logic, so that we are in a position to assist you. If your thinking is "off", we need to know first what it is before we can put you on the right track.
  5. Jul 10, 2014 #4
    my reasoning is first i choose 5 blocks from the 7 blocks. then since there 're 4 same colour red and 5 same colour yellow, so i divide it by 4! X 3!
  6. Jul 10, 2014 #5


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    You need to provide more information. Is that really the full problem statement? I don't see an interpretation that yields the answer 8. For example, if you end up choosing 4 red and 1 yellow, I'd say that 5 arrangements are possible, and if you end up choosing 3 red and 2 yellow, I'd say that 10 arrangements are possible.
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