Question 1001675
Variables x, y, z for low, medium, high conc. materials.

{{{system(x+y+z=64,z/y=2,(20x+30y+75z)/64=50)}}}
Simplify the system to something convenient or comfortable.
{{{system(x+y+z=64,z=2y,4x+6y+15z=640)}}}


Obvious substitution for z.
{{{highlight(system(x+2y=64,x+36y=640))}}}
Using elimination subtracting the '640' equation from the '64' equation,
{{{highlight_green(y=288/17=highlight(16.94*liters))}}} for the 30% material.


You can determine x for the 20% material and the z for the 75% material yourself.