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Math Help - Ok what am I doing wrong here? 2 questions.U

  1. #1
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    Ok what am I doing wrong here? 2 questions.U

    Hello.
    This is under probability in my book where you can use counting or combo or permutation rule.

    QUESTION 1
    HOW many 5-digit USA numeric postal(zip) codes are possible if the same 5 digits can be repeated?
    ANSWER is 100,000.

    If this is the counting rule. I get 3125???

    QUESTION 2
    How many different 3-digit ID tags can be made if the digits can be used more then once??
    So n=10, r=3 10Cr = 120 ANSWER in book is 1000
    Part B of this question says If the 1st digit must be a 5 and repeats are NOT ALLOWED?
    n=10, and reduce r by 1 so r=4?
    10P4 = 5040 ANSWER is book is 72.

    What am I doing wrong? Why on earth is this so hard I don't get it. I LOVE MATH. Stats as I see is another story.
    Any help would be great,
    thanks
    Jo
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  2. #2
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    Re: Ok what am I doing wrong here? 2 questions.U

    Hey Jo.

    I have same problem with statistics and probability. anyways...

    q1) Uses the Multiplication Rule: Consider each number on the plate as a position, or a bin to put a number into. How many numbers can you choose from to put in each position/bin?
    I get (10)*(10)*(10)*(10)*(10)

    q2)
    (a) same again: (10)*(10)*(10)

    (b) here the first position must be 5 --> 1 choice
    second position can be any of remaining 9 digits --> 9 choices
    third position can be any of remaining 8 digits --> 8 choices
    Mulitiplication Rule: (1)*(9)*(8) = 72.
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