I'm having an issue with this same exact problem. This is the DE that I got from the initial problem.
\frac{\mathrm{d} C}{\mathrm{d} t} = \frac{4gal}{min}(5ppb)-\left( \frac{2ppb}{200gal+(2gal)t}\right )C
Is this anywhere close to the equation that I should be finding? I've solved the...