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Counting techniques help

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Find your uni forum to get talking to other applicants, existing students and your future course-mates 27-07-2015
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    Can someone please explain to me how this concept works and how it applies to this question:

    n(AuB) = n(A) + n(B) - n(AuB)

    The question:

    How many three digit codes can be made using the digits 0,1,2,3,4,5 if there are no restrictions on using a digit more than once?

    How many of these codes:

    f)start with a 3 or end with a 4?

    My solution:

    1x6x6 + 6x6x1 - (1x6x1) ---> answer is correct but i dont understand the n(AuB) part so please can you help me, i understand that the events are mutually exclusive so yeah..

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    Let A = start with 3, B = end with 4
    n(AuB) = n(A) + n(B) - n(AnB).

    Does that help?


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