Flow differences?

Question

consider a reservoir with a volume of 2 billion cubic meter and an initial pollutant concentration of .005g/m^3. There is a daily inflow of 100 million m^3 of water with a pollutant concentration of .0005g/m^3 and an equal daily outflow of the well-mixed water in the reservoir. How long will it take to reduce the pollutant concentration in the reservoir to .001 g/m^3.

Answer 1

I setup an Excel spreadsheet and itterated the problem.  The answer I got was 160 days.

Here are the fomulas except for the initial condition in row 1

ColA: 2000000000

ColB: 100000000

ColC: =0.0005*B2

ColD: =D1-C2

ColE: =D2/A2

D1 has 100000000

copy and paste row 2 (A-E) down until you read 0.001 in cell E161

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