If the town decreases at a rate of 8% per year, it would take 6 years.
100% - 8% = 92%
We would multiply each population per year by 0.92 as the town is decreasing in population by 8% per year
1st year: 18,000 • .92 = 16,560
2nd year: 16,560 • .92 = 15,235.2
3rd year: 15,235.2 • .92 = 14,016.384
4th year: 14,016.384 • .92 = 12,894.72
5th year: 12,894.72 • .92 = 11,863.1424
6th year: 11,863.1424 • .92 = 10,914.091
10,914 is less than 11,000 meaning it would take 6 years for the population to be fewer than 11,000 if the town is decreasing in population at a rate of 8% per year