Thomas Malthus is famous for writing that, if left unchecked, the world's human population would double every 25 years. He wrote this in 1798, when the population was about 0.9 billion.

(a) Write a formula that would give the population (in billions) t years after 1798 if it really had doubled every 25 years.

(b) According to your formula, what would be the population in 2025 (at time t = 227)? Give your answer to the nearest billion.

(c) What is the annual percentage growth rate of your function? Give an answer correct to three decimal places (as in 1.234%).