the population of a city in 2005 is 18,000. by 2010, the city's population is 45,000. economists have determined the that the population growth follows an exponential model. if they are correct, what is the projected population for 2015?

There could be several answers.

You could add 27,000 each time (72,000)

or you could find what percentage 18,000 is of 45,000, then use that percentage of increase to figure out the next number for 45,000 (72,000)

Surprisingly, you will end up with 72,000 in both of these configurations.