This problem is fairly easy to solve using the mean time to failure logic in Probability ( if you are familiar with random variables):
Pr(Girl) = Pr(Boy) = 1/2
Assume a Random variable R denoting the number of girls before boys
Expectation® = (1/probability of event ) -1
which is equal to 1 (since 1/1/2 =2 and 2-1 = 1) So, we can say that each couple expect to have one baby girl before getting a baby boy and hence the most probable assumption is each family has a boy and a girl, and therefore an equal number of boys and girls in town.
This brings to final answer as 1:1 i.e proportion is 1 or 1.00 .
For more info watch this lecture: