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