Question 435553
The following list gives the sample space (assuming no multiple births!)
BBB
BBG
BGB
BGG
GBB
GBG
GGB
GGG
 
There are 7 sample points where there is at least 1 girl.  Of these seven , there are 6 elements where there is at least 1 boy.  Hence the probability that one is boy given that one is a girl is 6/7.