Because the world’s population has grown at an average rate of 1.9% per year since 1945, it follows that the world’s population has been growing by a constant factor of 1.019 since 1945. If the world’s population in 1975 was about 4 billion, in 1976 t he world’s population would have been about 4(1.019); in 1977 t he world’s population would have been about 4(1.019)(1.019), or 4(1.019) 2; and so forth. Therefore, the world’s population, P(t), t years since 1975 could be represented by the function P(t)=4(1.019)t.