<PRE><B>Question 178448</B>
let x and y be the 30% and 70% solutions respectively
:
x+y=70......eq 1
.3x+.7y=.5(70)...eq 2
:
take eq 1 and rewrite it to y=70-x and plug it into eq 2
:
.3x+.7(70-x)=.5(70)
:
.3x+49-.7x=35
:
-.4x=-14
:
{{{highlight(x=35)}}}liters of 30% solution
:
{{{highlight(y=70-35=35)}}}liters of 70% solution
:
we really could have come up with this by common sense. 50% is half way between 
30% and 70% so to have equal portions makes total sense</PRE>