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Interpret change in exponential models: changing units

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After the last ice age began, the number of animal species in Australia changed rapidly.
The relationship between the elapsed time, t, in years, since the ice age began, and the total number of animal species, Syear(t), is modeled by the following function:
Complete the following sentence about the rate of change in the number of species in decades.
Round your answer to two decimal places.
Every decade, the number of species decays by a factor of  
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3/5
  • a simplified improper fraction, like 7/4
  • a mixed number, like 1 3/4
  • an exact decimal, like 0.75
  • a multiple of pi, like 12 pi or 2/3 pi