Answer: 10,000
Step-by-step explanation: here's how I got my answer:
Since we are finding the decrease from 2005-2015, we need to multiply 1/20 by 10, since the timespan is 10 years worth of decreasing.
So, 1/2 * 10 =1/2
So, since we have 1/2 as the overall decrease in the population within the 10 years, we divide 20,000 by 1/2 or 20,000 by 2, and we get 10,000. I hope this helps.