Let's say we have a village with just 4 families, 1 for each possible case. The names of the boys are b0,b1,b2,b3 and the names of the girls are g0,g1,g2,g3 (as

Let's say we have a village with just 4 families, 1 for each possible case. The names of the boys are b0,b1,b2,b3 and the names of the girls are g0,g1,g2,g3 (as no two names are the same): b0, b1 b2, g0 g1, b3 g2, g3 If we draw a random family that has at least one boy, we get all the families except the last one. And 2/3 of those have a girl in them, as opposed to 1/3rd that doesn't. So its 66% to find a girl. However, if I ask you about a particular boy, I could ask about b0, b1, b2 or b3. b0: sibling is b1 b1: sibling is b0 b2: sibling is g0 b3: sibling is g1 So for half of the possible names I choose, the sibling is a boy, and for half it is a girl. Assuming the name was chosen at random from the existing names of the boys in the village, that makes the chance 50% to find a sibling girl again.