
07/07/2004, 04:07 AM
#99
Originally Posted by ExtraOrdinaryJo
OK, another old (and easy) one:
A couple have two children. You knock at their door, and one of their two children, a girl opens the door. What is the probability that the other child is also a girl?
Originally Posted by ExtraOrdinaryJo
Correct! So the answer is 1/3....
Sorry ExtraOrdinaryJo, but I am not with you on this one:
If you roll a dice, what is your chance of getting 6? 1/6
But: if you roll a dice and get a six, what is your chance of getting a 6 again? Actually, again 1/6, because the result of your first try has no influence on your second try. After all, the dice does not know or care what you had in the first try.
The situation is the same in your example: the fact that the family already has a girl has no influence on the question whether the second child is a boy or a girl, so the chance that the other child is a girl too is 50% (close to), in my view, or am I wrong?
The situation is different when you ask, "If a family has two kids, what is the chance that both are girls?" Then the correct answer would be 1/4 (25%), because there are 4 different combinations (as ChickDance pointed out), and oly one is girlgirl. But if we already know one of the two is a girl, then only two possiblities are left, boy or girl, 1/2 (50%).
So ToolkiT was right, I guess...


