Answer
$x=12; y=18$
Work Step by Step
Let $x$ and $y$ be the number of liters in the $25 \%$ solution and $50 \%$ solution, respectively.
The system of two equations is:
$x+y=30 \\ 0.25 x+0.5 y=12$
Multiply the first equation by $-5$ and then add the new equation to equation $2$.
$-0.5x-0.5y+0.25x+0.5 y=-15+12 $
The yields $x=12$
Substitute the value of $x$ into the first equation to get the value of $y$.
$12+y=30 \implies y=18 $
Thus, $x=12; y=18$
