Probability Puzzle

ANSWER: 1/3

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At first, I immediately thought that the chance was 1/2 although since it isn't, I used my reasoning critically skills and tried to look at the problem at a "different angle". Here's my math work to prove that the chance is 1/3.

             Before the girl was born, here are all of the possible outcomes of the children if the girl wasn't born yet.

1. Boy & Boy
2. Boy & Girl
3. Girl & Girl
4. Girl & Boy

In the text, it states that one of the couple's children is a girl so you have to cross out the 1st possible outcome as that possibility is gone as one of the kids is a female.

1. Boy & Boy
2. Boy & Girl
3. Girl & Girl
4. Girl & Boy

Since we have to find out the possibility if all of the kids are girls, you have to cross out the ones with girls with boys. 

1. Boy & Girl
2. Girl & Girl 
3. Girl & Boy

So there you go, there is a 1/3 chance that Teanchi and Beanchi's children are all girls.