Maths problems (interesting ones)

[Alan Anderson]

Originally Posted by Kris Verdeyen

Actually, I'm the only one catching it - it's twice as likely that a girl will initally answer from a G/G family than from a B/G family, and there are twice as many B/G families. Therefore, the other sibling has a 50/50 shot of being girly.

No, you're counting families too soon. Only half of of the boy/girl families had a girl answer the door, so the others don't meet the criterion for this problem.

None of the boy/boy families are included. Half of the boy/girl families are included. All of the girl/girl families are included. But once a girl answers the door, you have to completely forget all the families that didn't get included. You can't count them, because they don't exist in the set of familes that did have a girl answer the door.