Question

A city of population 2.0 million is expected to experience a 15% decrease in population every 10 years.

Answer 1

Not for sure on what you are looking for. I hope this will help.

2.0 million in population
15% decrease every 10 years

Make sure you turn 15% into a decimal .15
So...
20,000,000 * .15 = 3,000,000