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Basic probability

  1. Dec 19, 2008 #1
    1. The problem statement, all variables and given/known data

    How many 4-digit patterns are there in which all the digits are different?

    2. Relevant equations

    10!/6! ??

    3. The attempt at a solution

    As shown above, I used 10!/6! and I get 5040. I get the same answer of course when I use 10x9x8x7. But my book shows the answer as 4536. What am I missing?
  2. jcsd
  3. Dec 19, 2008 #2


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    Homework Helper

    :wink: the numbers that you want to arrange do not start with a 0 :wink:
  4. Dec 19, 2008 #3
    Gadzooks! It never crossed my mind..
  5. Dec 22, 2008 #4
    the total number of combinations of 4 digit numbers you can get is 9^4. Now just count how many contains same digits.
  6. Dec 22, 2008 #5


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    Staff Emeritus
    Science Advisor

    Choose any of the 9 digits (1 through 9) for the first digit. That leaves 9 digits for the second (because you are now allowed to use 0), 8 for the third digit, and 7 for the fourth.
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