# Math Help - discrete prob

1. ## discrete prob

Hi
Dopes somebody knows the answere for that kind of question?
Determine the number of vectors(x1,...,xn) such that each xi is either 0 or 1 and sum(n,i=1)xi>=k
Thank you

2. Assuming that the value of n is fixed and $0 \le k \le n$, then you are counting the number of bit-strings of length n that have the sum of their enters great than or equal to k. The bit-strings with k ones and (n-k) zeros fit that description. How may of those are there?
Do it for each j, $k \le j \le n.$