population decline by yearly factor decrease

Question

Trying to help my daughter with a word problem. Here's the question:

Every year the population of Springfield decreases by a factor of 1/6. If the population of Springfield on Jan 1, 1980 was 3,521, what was the approximate population on Jan. 1, 2000?

Answer 1

Hint:

This is a geometric sequence, after $n$ years the population is $3521\cdot\left(\frac{5}{6}\right)^n$.