Rosario is an ecologist who studies the change in the penguin population of Antarctica over time. The relationship between the elapsed time ttt, in years, since Rosario started studying the population, and the number of penguins, P(t)P(t)P, left parenthesis, t, right parenthesis, is modeled by the following function:

P(t) = 21,00(4/7)^(t/3)

Complete the following sentence about the rate of change in the penguin population.
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The population of penguins loses 3/7 of its size every _ years.