Question 1204800
Viewpoint is usually solute.  Your question is only about solvent.  So for focusing on the percents water:


v of the 70%water
20-v of the 30% water
Want 60% water



{{{70v+30(20-v)=60*20}}}

-

{{{70v-30v=60*20-30*20}}}

{{{(70-30)v=20(60-30)}}}

{{{highlight_green(v=20((60-30)/(70-30)))}}}

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You could use a system of equations if you really want, but obviously  something like v+x=20, whatever you want to choose for variables...



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SYSTEM of two equations
v of the 70%
x of the 30%
{{{v+x=20}}}


The "pure water", {{{70v+30x=60*20}}}

or simplifying,
{{{7v+3x=60*2}}}
{{{7v+3x=120}}}


and a system can be  {{{system(7v+3x=120,v+x=20)}}}.