I don’t understand why we need to multiply every item by C ?
My understanding is that C can represent both the number of families and the number of boys. Since every family is guaranteed to have exactly one boy, C=1. The number of girls and the probability of having n girls scales linearly with C, hence the multiplication of every item by C.
C is multiplied with every item because it represents the total population of the country.
C represents the number of couples. So for 1 girl it is
No of couples * probability or one boy * probability of 1 hgirl