Apopulation numbers 20,000 organisms initially and decreases by 7.9% each year.
suppose p represents population, and t the number of years of growth. an exponential model for the population can be written in the form p = ab^t , where
p=

  P = 20000·0.921^t

Step-by-step explanation:

In the exponential model given, "a" is the initial value, 20000, and "b" is the year-to-year multiplier, or growth factor. It is 100% - 7.9% = 92.1% = 0.921. So, your exponential model can be written ...

  P = 20000·0.921^t

you didn't post any answer choices, so its not 100% clear what your teacher is looking for

however, log(15/7) is equivalent to log(15) - log(7)

i'm using the rule log(x/y) = log(x) - log(y)