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Re: A particular natural history museum has three rooms [#permalink]
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total ways to arrange 9*8*7= 504
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Re: A particular natural history museum has three rooms [#permalink]
why not 9c3= 84 ways
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Re: A particular natural history museum has three rooms [#permalink]
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Basic principle of counting method

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Re: A particular natural history museum has three rooms [#permalink]
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gajala wrote:
why not 9c3= 84 ways


I also used to struggle with when to just write out ___ X ___ X ___ and when to use the formula. In this case, consider what the formula means "9 choose 3" But we are not really choosing 3. We are choosing 1 per room, or better said, we are choosing any of them for a room and then any of the left ones and then a final time. Perhaps someone else can give an example of this same question reworded so that the formula would be needed? Perhaps if we had 9 fossils, one display room and this one room was going to display 3 fossils at the same time?
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Re: A particular natural history museum has three rooms [#permalink]
If the question was to just select 3, then one could use 9C3 but since this can be arranged differently in the three different rooms, it is 9P3 = 504
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Re: A particular natural history museum has three rooms [#permalink]
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Re: A particular natural history museum has three rooms [#permalink]
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