Answer: 395 birds
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Work Shown:
The formula to use is
A = P*(1+r)^x
where,
P = 12000 is the initial population
r = -0.15 represents the growth rate in decimal form; which is really a decay/decrease due to r being negative
x = 21 days have passed by (3 weeks = 3*7 = 21 days)
These values lead to...
A = P*(1+r)^x
A = 12000*(1+(-0.15))^21
A = 12000*(1-0.15)^21
A = 12000*(0.85)^21
A = 12000*0.03294560142183
A = 395.34721706196
A = 395
