The question is a little unclear, I think by "maintains" they mean "starts out with." Also it's unclear whether the new plants are included when calculating that 10% get sold. I'll assume not. Additionally, it's not clear whether 50,000 should be the number of plants for year zero or for year one! I'll assume one.

So we can say

f(1) = 50000

for all integers n > 1: f(n) = 0.9 * f(n-1) + 4000

I get f(20) approximately equals 41350.85