Question

How many bit strings of length 9 contain exactly 5 ones

Answer 1

The computer science tutors would be far more able to help you with this problem than the biology group. Posting your question there, rather than in this forum, would make it easier for them to find it.

Answer 2

It's finding the number of distinct ways of arranging four 1s and five 0s.
That is (9!/4!5!) = [10x9x8x7x6x5x4x3x2x1]/[(4x3x2x1)(5x4x3x2x1)]=126

There are 126 bit strings of length 9 with exactly five 0s.

Answer 3

And yes this belongs in computer science :D