"George Patterson" wrote in message news:Cc6uh.1262$FN1.1231@trnddc08...
Pszemol wrote:

Can you assemble an equation to solve this problem ? :-)

Us a variation of compound interest calculations. Let n be the number of water
changes, N1 be the initial nitrate level, N2 be the desired nitrate level, and P
be the percentage of water changed. Then

n = log(N2 / N1) / log(1 - P)

Solving for N1 = 100, N2 = 10, and P = .1, get get 21.85434512 changes.
Solving for N2 = 20, we get 15.2755317.

Very good! We should name you a Group Math Wizard