4. A group of individuals containing b boys and g girls is lined up in random order?that is, each of the (b + g)! permutations is assumed to be equally likely. What is the probability that the person in the ith position, 1 ≤ i ≤ b+g, is a girl?
The answer is g/(b+g).
Let's denote C(n,m)=n!/(m!*(n-m)!).
Because the b boys and g girls are lined up in random order. The ith position and the jth position make no difference. Thus for the ith position, we have g/(b+g) chance to put in ...
A probability problem involving permutation and position is solved.