Answer:
4.47 years
Explanation:
Fill in the numbers and solve for t.
P = a·b^t
15390 = 19000·0.954^t
15390/19000 = 0.954^t . . . . . divide by 19000
0.81 = 0.954^t . . . . . . . . . . . . . simplify
log(0.81) = t·log(0.954) . . . . . . take the log
t = log(0.81)/log(0.954) . . . . . . divide by the coefficient of t
t ≈ 4.47
It will take about 4.47 years for the population to drop to 15390 organisms.
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A graphing calculator can be another way to solve a problem like this.