Question 817395
{amount of antiseptic}/{amount of antiseptic solution}={concentration of resulting solution}

Let x = amount of the 50% solution to use, in LITERS.


{{{highlight_green((50x+4*20)/(x+4)=30)}}}
Divide both sides by 10,
{{{(5x+4*2)/(x+4)=3}}}
{{{5x+8=3(x+4)}}}
{{{5x+8=3x+12}}}
{{{2x=12-8=4}}}
{{{highlight(x=2)}}}