The population of Collin County, which follows the exponential growth model, increased from 491,675 in 2000 to 782,341 in 2010. Find the exponential growth rate, k.

p=ak^t

p1=491272*k^0

p2=782341*k^10

782341/491272 = k^10

k=1.047629

growth rate is 4.7629%

2012: predicted population is 491272*1.047629^12=858,642 people

1245863=491272*1.047629^t

1.047629^k=2.5363599

ln both sides is k ln 1.0476=ln 2.5363

t=20.000 years.

Rule of 72 would say 72/4.76 or 15.12 years for doubling

14.90 years

2017 predicted is 491272*1.0476^17 or 1083552, or more than actual, so the rate of growth is slowing down.