Question 714702
A very routine exercise.  Let x = volume of the 20% acid to use and y = volume of the 50% acid to use.


{{{(20x+50y)/30=28}}} AND {{{x+y=30}}}


The numbers were picked to make the arithmetic easier to do.
Managing the percentage equation,
(2x+5y)/3=28   just by removing factor of {{{1=10/10}}} from left side;
2x+5y=3*28
2x+5y=84


System to solve is this:
{{{2x+5y=84}}}
{{{x+y=30}}}


Possible next step is
{{{2x+5y-2(x+y)=84-2*30}}}
{{{5y-2y=24}}}
{{{3y=24}}}
{{{highlight(y=8)}}} liters for the 50% acid.

You can continue from here...


Also notice, if 30 liters mixture is to result, as described, then this must be some huge bottle containing the x liters of the other acid!