Question 793645
x = liters of 25%
y = liters of 35%
z = liters of 75%
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Restriction given, z=3y.  We can rewrite the variable assignments:
x = liters of 25%
y = liters of 35%
3y = liters of 75%


Account for concentration of the intended acid solution:
{{{(25x+35y+3y*75)/64=55}}}
and simplify,
{{{(25x+(35+3*75)y)/64=55}}}
{{{(25x+260y)/64=55}}}
{{{25x+260y=64*55}}}
{{{5x+52y=64*11}}}
{{{5x+52y=704}}}


Account for volume:
{{{x+y+3y=64}}}
{{{x+4y=64}}}


Work with the system of the two equations in x and y:
{{{highlight(5x+52y=704)}}}
{{{highlight(x+4y=64)}}}
Once x and y solved, you can compute z.


The steps from here are not provided but are omitted intending for you to do them.  Tell me if this is troublesome.  Analyzing the problem and coming up with the system of equations is the hardest and most important part.