Answer:
5 years
Explanation:
Exponential decay:
y = a(1 - r)^x
y = amount at x time
a = initial amount
r = rate of decay as decimal
x = time in years
y = 20000(1 - 0.09)^x
13000 = 20000(0.91)^x
13/20 = 0.91^x
log (13/20) = x log 0.91
x = log 0.65 / log 0.91
x = 4.6
It takes 4.6 years for the population to decrease to 13,000.
Answer: 5 years