Answer
$1.3\times 10^3 mL\space soln$
Work Step by Step
The volume of chlorine solution in $mL$ can be determined as follows:
First of all, we convert the gallons of water into milliliters
$Water \space volume=2\times 10^4 gal\times 3.79L \times 1000=7580\times 10^4mL$
Now the mass of chlorine is given as
$mass \space of \space chlorine=7580\times 10^4mL\times \frac{1g\space H_2O}{1mL\space H_2O} \times\frac{1g \space Cl_2}{10^6\space g H_2O}=7580\times 10^{-2}g $
Given that the solution contains 6 percent chlorine by mass
$\implies Volume \space of \space chlorine \space solution=7580\times 10^{-2}g \times \frac{100 g \space soln}{6g}\times \frac{1mL\space soln}{1g\space soln}=1.3\times 10^3 mL\space soln$
