[AmishDude]

I suspect said problem was hastily put together so as to make all of the arithmetic uncomplicated. You are right, the answer they are looking for is 12. Others on the thread are right, it makes no sense, unless 4 out of 5 of the births are lethal and the one out of a million estimate includes both lethal and non-lethal births.

Of course, if you remove the stable population restriction the problem can be made to make sense, but that would involve...gasp...variables.

And, this seems to model asexual reproduction. Kinda like the Fibonacci rabbits.

[AmishDude]

Also, the 4/5 births death assumption would mean they aren't included in the average 10-year lifespan.