Answer
$20\;L$
Work Step by Step
Let the amount of antifreeze to be added is $x\;L$.
We can write.
$60\%$ of $20\;L+x=80\%$ of $(20\;L+x)$
The equation form is
$\Rightarrow 12+x=16+0.8x$
Subtract $0.8x+12$ from both sides.
$\Rightarrow 12+x-(0.8x+12)=16+0.8x-(0.8x+12)$
Simplify.
$\Rightarrow 12+x-0.8x-12=16+0.8x-0.8x-12$
$\Rightarrow 0.2x=4$
Divide both sides by $0.2$.
$\Rightarrow x=\frac{4}{0.2}$
$\Rightarrow x=20$.
