Answer: 2/3 – Initially, there are 4 equally likely options for the children’s genders (Boy=B, Girl=G): BB, BG, GB, GG. Since you do not know which child is certainly the boy, the combinations BB, BG and GB are all still viable and equally likely. Therefore in 2/3 cases the other child is a girl.
“You make a dash for the exit…”
TOM ROCKS MATHS 