## Sunday, October 30, 2011

### Boys vs. Girls

Here is another interesting problem I was trying to address a while ago:

In a country in which people only want boys, every family continues to have children until they have a boy. If they have a girl, they have another child. If they have a boy, they stop. What is the expected proportion of boys to girls in the country?

Apparently, this problem was originally posted by Google, as this post suggests. Here is a link to another stream, trying to tackle the problem. I will try to provide a solution that (in my opinion, of course) looks easier to comprehend.

At any given moment of time, the total number of couples (with children) can be divided into two categories CTotal = C1boy + Cno boys, where C1boy - number of couples with 1 boy (this is actually the limit, as it is stated in the problem) and Cno boys - number of couples having only girls (and thus, still trying to have a boy). We don't count the couples with no children as they don't add any value to the calculations below.

The number of boys in this case (or at any given moment of time) is Nb= C1boy.

The number of girls is Ng=N1⋅C1boy + N2⋅Cno boys, where N1 - average number of girls in a family with 1 boy and N2 - average number of girls in a family with no boys. Let's find these averages.

The probability for a family with one boy to have 1 girl is P(1 girl & 1 boy) = (1 ⁄ 2)⋅(1 ⁄ 2)
The probability for a family with one boy to have 2 girls is P(2 girls & 1 boy) = (1 ⁄ 2)⋅(1 ⁄ 2)⋅(1 ⁄ 2)
...
The probability for a family with one boy to have n girls is P(n girls & 1 boy) = 1 ⁄ 2n+1

So, the average number of girls in a family with 1 boy is (find the formula for this series here) N1=E(X) = ∑m⋅P(m) = ∑m ⁄ 2m+1 = (1 ⁄ 2)⋅∑m ⁄ 2m= (1 ⁄ 2) ⋅ (1 ⁄ 2) ⁄ (1 - 1 ⁄ 2)2 = 1.

Now...

The probability for a family with no boys to have 1 girl is P(1 girl) = 1 ⁄ 2
The probability for a family with no boys to have 2 girls is P(2 girls) = (1 ⁄ 2)⋅(1 ⁄ 2)
...
The probability for a family with no boys to have n girls is P(n girls) = 1 ⁄ 2n

The average number of girls in a family with no boys is N2=E(Y) = ∑k⋅P(k) = ∑k ⁄ 2k = 2.

As a result Nb ⁄ Ng= C1boy ⁄ (C1boy + 2⋅Cno boys). This expression is equal to 1 only when Cno boys=0, i.e. when all the families reach the target. However, if Cno boys= C1boy then Nb ⁄ Ng=1 ⁄ 3.