I have two children. One of them is a boy. What are the odds the other one is also a boy?If you're a human living in the world, three things are probably true of you vis-a-vis this puzzle. 1) You've heard it before, 2) you got it wrong the first time you heard it, and 3) the correct answer still seems wrong to you.
The correct answer, if you've never encountered it, is based on the following a priori: there are four equally-likely ways to have two children:
- a boy followed by a girl
- a boy followed by another boy
- a girl followed by a boy
- a girl followed by another girl
Mrs. Transient Gadfly will tell you that Mr. Transient Gadfly's position on all questions of this nature is that it is not a math question, it is a language question. And, moreover, it is an ill-posed one. The nature of how poorly this question is posed is laid bare by the variation linked above:
"I have two children, one of whom is a boy born on a Tuesday. What's the probability that my other child is a boy?"If you follow the logic of the original problem (which, being that I am a human in whose true nature you will find the compunction to write this blog, I did) you'll write out all the days of the week your first child could be born, followed by all the days of the week your second child could be born, look at all of those that have a boy born on a Tuesday in them, count the number of those that have a second boy, and come up with the answer (it's 13 in 27, if you write out the table. Do not write out the table). If you are literally anyone else in the world, you will come up with a much better answer: 1 in 2. The crux of the issue, which the linked article almost hits on but then fails to, is that there is no universe in which the given answer (13 in 27) is correct. It would require the asker of the question to randomly chose a day of the week and a gender, and then only pose the question if he or she had a child that matched those criteria.
(Here is one of those moments where Transient Gadfly has an existential crisis about the nature and purpose of The Odds Are One: should I explain why what I just said is true? It would take, like, seven paragraphs and still nobody reading would understand the logic. I'm not going to do it this time. You'll just have to take my word on this one).
If you're anyone else, you look at that question and understand the only way someone would pose the question: he or she randomly chose one of his or her children, and listed two characteristics of that child: his gender, and the day of the week of his birth. And you will come up with the correct answer to the question, because when you randomly chose one of your two children, the gender of the other one is a coin-flip. So, you might well ask, what is the difference between the original question posed by Martin Gardner and the question involving the day of the week? And the answer is, absolutely nothing. There is no way to tell, from the way it is stated, whether the asker, a parent of exactly two children, randomly chose one of his progeny and told you his gender, or a parent of exactly two children, of whom at least one is a boy, told you that fact. And it matters, because in the former case it's a 50% shot that the other child is a boy, and in the latter it's a 33% chance.
I leave you with a link to an XKCD cartoon, because it's literally impossible to make this point better than he has here.