Answer
$125$ liters
Work Step by Step
Convert $28\%$ to a decimal as $0.28$, and set it equal to $C$ in the given formula. Then distribute and combine all the like terms, and isolate the $x$-term and divide to solve for $x$.
$C=\frac{x+0.1(500)}{x+500}$
$0.28=\frac{x+50}{x+500}$
$0.28(x+500)=x+50$
$0.28x+140=x+50$
$140=0.72x+50$
$0.72x=90$
$x=125$