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December 27th, 2017, 04:41 AM   #1
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The number of choices

What is the number of choices to compose a number with the 3 digits: 1, 3, 9? (The digit need no to be repeated.)
What is the formula? I forget it, so if you can write it, it would be helpful.
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Last edited by skipjack; December 27th, 2017 at 09:15 AM.
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December 27th, 2017, 05:02 AM   #2
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As the digits cannot repeat, the first digit can be either 1, 3 or 9 (3 options). Second digit can be one of the remaining 2 (2 options) and the final digit is the left over one (1 option). Hence total number of choices you have is 3*2*1 = 6.

139
193
319
391
913
931
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December 27th, 2017, 05:42 AM   #3
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Nice. Thanks...
it is n!, I think...

Last edited by policer; December 27th, 2017 at 05:49 AM.
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December 27th, 2017, 08:10 AM   #4
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Yes, you got it!.
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December 27th, 2017, 08:47 AM   #5
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Saying the "digits need not be repeated" (as in #1) is not the same as saying "the digits cannot be repeated" (as in #2).
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December 27th, 2017, 09:19 AM   #6
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The wording in post #1 is "need no to be", which is closer to "need not to be" than "need not be".
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