Question

In 2010, the population of a town was 8500. The population decreased by 4.5% each year.

(a) Write an equation to find the population of the town t years after 2010.
(b) In what year will the population of the town be 7000?
Show your work.

Answer 1

This is a problem in exponential decay.

a) If a town's population decreases by 4.5% every year that also means that the town's population decreases by a factor of .955 each year. (1 - .045 = .955)

So, after 5 years, the town's population is:

8,500 * .955^5 which equals 6,752.

So, basically, after t years, the town's population equals

8,500 * .955^t where t is the number of years that have passed since the year 2010.

b) population = 8,500 * .955 ^ (number of years since 2010)

7,000 / 8,500 = .955 ^ (number of years since 2010)

0.8235294118 = .955 ^ (number of years since 2010)

To solve for (number of years since 2010) we take logs of both sides

log (0.8235294118 ) = number of years since 2010 * log(.955)

-0.0843208857 = number of years since 2010 * -0.0199966284

-0.0843208857 / -0.0199966284 = number of years since 2010

4.2167551422 = number of years since 2010

So, population = 7,000 when the year is 2014.2167551422

Or about 2.6 months into 2014

(YES, it's just that "easy") LOL

Explanation: