the correct answer to this question what is the population size at year 4, given
a birthrate of 3, is 81
the reason for that is that if the birthrate is 3
we start off with 1 and then at year one we get 3
at year 2 each of those 3 has 3
at year 3 each of those has 3
and at year 4 we multiply that by another 3
that's equal to 81