You have to write the function of exponenetial growth and find its growth factor. If N(t) - population after "t" years, and "b" is a growth factor, then your function is N(t) = N0 x btwhere N0 = 800 - initial population. If the population doubles every 7 years, that means N(7) = 1600, and we can write 1600 = 800 b7 or 2 = b7ln Taking logaritms of both ides of the equation, we can find b: ln b = ln2/7 or b = exp(0.099) = 1.104 After 25 years N(25) = 800 (1.104)25 = 9491 - answer.
