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Using the digits $1$, $2$, $3$, $7$, $8$, $9$, and $0$, how many $4$-digit numbers can be created that are greater than $3718$?

My answer is : $(360)(1)(12)(40)= 413$

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    $\begingroup$ How did you get your answer? What is your thought process? $\endgroup$ – Sean English Jan 13 '16 at 2:45
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    $\begingroup$ To make sure: Can digits be used repeatedly? $\endgroup$ – OnoL Jan 13 '16 at 2:47
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    $\begingroup$ With those digits, how many $4$-digit numbers can be created that are greater than $3000$? (I wouldn't expent the answers to be the same, because you can create some numbers between $3000$ and $3718,$ for instance $3298.$) $\endgroup$ – bof Jan 13 '16 at 2:50
  • $\begingroup$ Did you mean $360 + 1 + 12 + 40 = 413$? $\endgroup$ – N. F. Taussig Jan 13 '16 at 12:45
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In the event that digits are not allowed to be repeated:

Let us break this into cases: - Case 1: The number is greater than or equal to $4000$

  • Step 1: Pick the first digit. Options are $7,8,9$ for a total of $3$ options

    • Step 2: Pick the second digit. Options are any of $1,2,3,7,8,9,0$ except what was picked in step 1 for a total of $6$ options.

    • Step 3: Pick the third digit. Options are any of the available digits except those already picked for a total of $5$ options.

    • Step 4: Pick the fourth digit. Options are any of the available digits except those already picked for a total of $4$ options.

    • This gives a total of $3\cdot 6\cdot 5\cdot 4$ numbers in this case

  • Case 2: The number is less than $4000$

    • Case 2a: The number starts with $371\square$. There is only one possibility: $3719$

    • Case 2b: The number starts with $37\square\square$ and the third digit is larger than $1$. There are $3\cdot 4$ possibilities here

    • Case 2c: The number starts with $3\square\square\square$ and the second digit is larger than $7$. There are $2\cdot 5\cdot 4$ possibilities.

This gives a final grand total of $3\cdot 6\cdot 5\cdot 4 + 1 + 3\cdot 4 + 2\cdot 5\cdot 4=413$ numbers.

Your mistake was that the number you counted was instead the number of numbers greater than $3000$ that use the available digits, but not the number of numbers greater than $3718$. There are numbers that you counted between $3000$ and $3718$ that you shouldn't have.

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  • $\begingroup$ I got it thanks for helping me out $\endgroup$ – MC UO Jan 13 '16 at 5:18

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