CHALLENGE - How can f(x) = f'(x) ?

If f(x) represents the growth of a population (bacteria, people, pond lilies, etc.) and x is a measure of time, then f(x) is a function depicting population as a function of time, and f'(x) is the rate of change of the population as a function of time - How can f(x) be equal to f'(x) ? Is there a way to fix the problem ?