SOLUTION: How many four-digit numbers can you form from the numbers 1,2,3,4,5,6,7,8, and 9 (a) if you use each digit only once? (b) if you can repeat each digit.
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Question 231623: How many four-digit numbers can you form from the numbers 1,2,3,4,5,6,7,8, and 9 (a) if you use each digit only once? (b) if you can repeat each digit.
Answer by solver91311(24713) (Show Source): You can put this solution on YOUR website!
a. 9 choose 4:
 = \frac{n!}{k!(n-k)!})
 = \frac{9!}{4!(9-4)!})
b. 9 ways to choose the first number times 9 ways to choose the second number...
In either case, you get to do your own arithmetic.
John
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