I like how no one has addressed a few issues.
1. Everyone had one baby, and that half of the babies where male, and the other half were female.
2. All the babies lived, which we all know from history class that child-birth before the 21th century had the possibility of being fatal (for the child & mother).
3. One final assumption is the one that in each generation, all the children lived to be old enough to have their own children which you can then reread 1 & 2.
1. Everyone had one baby, and that half of the babies where male, and the other half were female.
2. All the babies lived, which we all know from history class that child-birth before the 21th century had the possibility of being fatal (for the child & mother).
3. One final assumption is the one that in each generation, all the children lived to be old enough to have their own children which you can then reread 1 & 2.