SOLUTION: How many nine-bit strings contain exactly seven 1s?

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Question 416541: How many nine-bit strings contain exactly seven 1s?

Answer by richard1234(7193) About Me  (Show Source):
You can put this solution on YOUR website!
With seven 1's there must also be two 0's. Thus, the number of ways to make a nine-bit string is 9C2 or 9C7 (this is equivalent to choosing two or seven out of nine placeholders to be 0's or 1's). 9C2 = 9C7 = 36.