Two siblings are in a room behind a door, with
Toddsterpatriot waiting outside . One is sent out, and it is a girl.
Toddsterpatriot gets to meet her and shake her hand and exchange pleasantries. They don't just meet, they go on to get married and have 11 kids.
What is the probability that there remains a girl behind the door?
Correct answer: 1/3.
Probability = 0. It is neither a boy or girl. It's either a really ticked off woman or a man.... Waiting in a room for a couple of decades is going to do that to you while Todd is out having fun overpopulating the world with your sister who never writes to you.
Now,
Toddsterpatriot walks into a house with two rooms and a hallway. Two siblings are in the house, one in each room. He opens one door. He meets a girl and she spurns him; she plays for the wrong team, or something. What is the probability a girl is behind the other door?
1/2
But seriously folks, I think we are overthinking it. I don't think ordering plays a role.
At the start of this puzzle, the premise is that the genders of the two kids are statistically independent, namely 50:50 for each. Suppose you went through life meeting thousands of parents with their two kids visible or known. You will compile these statistics.
(Notation: OE logically means BG ∪ GB or
One of
Each. The order doesn't matter)
BB 25%
OE 50%
GG 25%
So, if for the first time you encounter one kid, a G, and you don't know the gender of the other kid, There can be many circumstances (The first kid is discovered behind a door, walking in the street..., the second kid is locked in a room, or in jail...) You will eliminate the BB possibility and resort to your statistics. You will see that the OE possibility is twice the GG possibility. That means the chances of a second G is half that for OE. So the chance is 1/3 that the second is a G and 2/3 that its an OE (in this case, a boy).
I think.
.
At the start of this puzzle, the premise is that the genders of the two kids are statistically independent, namely 50:50 for each.
And that's your answer. 50%.
So, if for the first time you encounter one kid, a G, and you don't know the gender of the other kid,
the genders of the two kids are statistically independent
You have two independent choices. 50/50.
You will eliminate the BB possibility and resort to your statistics.
Before you met the one kid, your choices were:
B1-G2
B1-B2
G1-B2
G1-G2
The one you meet can be B1 (boy first) or G1 (girl first).
When you meet a girl first, you eliminate B1-G2 AND B1-B2, or 50% of your original 4 possibilities.
Only 2 possibilities left......G1-G2 and G1-B2.
The odds that the second sibling is a girl is 1 out of 2.
The only way Fort Fun's riddle works out to 1/3 is if you have 4 children, two boys and two girls.
One girl leaves the house to meet you.
Now you have 2 boys and 1 girl remaining.
In this case, the odds of picking another girl is 1/3.