Take a look at the photo
May I have the method aswell for future reference?
Many thanks!
Ok what you do here is construct a probability tree for a single couple. Each couple probabilistically behaves the same so the overall ratio for the population is just the same as that for a single couple
Note that this all assumes the children don't themselves start forming couples (from other families hopefully) and themselves reproducing.
if you work through the thing you'll find that
$$Pr[\mbox{k girls, 1 boy}]=\left(\frac{1}{2}\right)^{k+1}$$
thus
$$P[\mbox{n children}]=\left(\frac{1}{2}\right)^{n}$$
For n children the ratio of girls to boys is
$$r[n]=\frac{1}{n-1}$$
and so you end up with a discrete probability distribution for certain ratios, i.e.
$$Pr\left[r[n]\right]=\left(\frac{1}{2}\right)^{n}$$
and zero for all other numbers. To make things a bit more concrete
$$\begin{align*}
&Pr[\infty]=\frac{1}{2} \\ \\
&Pr[1]= \frac{1}{4} \\ \\
&Pr\left[\frac{1}{2}\right]=\frac{1}{8} \\ \\
&\mbox{and in general for n>1} \\ \\
&Pr\left[\frac{1}{n-1}\right]=\frac{1}{2^n}
\end{align*}$$