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What is the formula to calculate population growth/rate?

How do I use the formula for this word problem:

A small country that had 20 million people in 1990 has experienced exponential growth in population of 4% per year since then. Write an equation that models this situation and use your equation to determine when the population will double.

I know my formula ends up being:

P(n) = 20 x (1.04)^n