hi guys.Can any one help me with this Q?

To control the population size and make a somewhat traditional population

happy at the same time, a government passes a law that requires couples to stop having

children once they have a son. Assuming that sons and daughters are equally likely,

the distribution of family size is geometric, with pdf given by

P(M = m) =1/2^m (Geometric distribution)

Interestingly, the average size of the family is 2, so the population size should stabilise. Since the standard deviation is radical 2, most families should have 5 or less children.

So how it got 5 children?