dont take this:bb
take this sample::bg,gb,gg

wi=event that ith child is girl.

p(w2) = p(w2|w1)p(w1)+p(w2|not w1)p(not w1)
= {gg}{gb,gg}+{bg}{bg}
= 1/3*2/3 + 1/3*1/3
=1/3