Predict the population?

Question

The population of Glenbrook is 375,000 and is increasing at the rate of 2.25% per year. Predict when the population will be 1 million.

Answer 1

Using the rule of 72 we can divide 72 by 2.25 = 32 years but this just predicts a doubling or 750,000.

The following I am not sure of.  You have 1 1/3 doublings, so we can change 72 to (72)* (1 1/3) = 96

Reapplying we would have  96/2.25 = 42 2/3 years.

That is my prediction, hopefully some one will come along that can do a better job.