Question 708706
{{{v}}}= known resulting volume, 75 liters
{{{p[1]}}} = low concentration material, 20%
{{{p[2]}}} = high concentration material, 50%
{{{p[t]}}} = the target concentration of mixture, 30%
{{{x}}} = how much low concentration material to use
{{{y}}} = how much high concentration material to use


INITIAL SYSTEM
{{{(xp[1]+yp[2])/v=p[t]}}}
{{{x+y=v}}}


Work with and possibly do other simplifications to the percent-concentration equation, 
{{{(xp[1]+yp[2])=vp[t]}}}


Use the volume sum equation to solve for either variable and substitute into the percentage equation and find the single variable there.  As general example, solve for y:


x+y=v, {{{y=v-x}}}.  Substitute.
{{{(xp[1]+(v-x)p[2])=vp[t]}}}, and solve for x
{{{xp[1]+vp[2]-xp[2]=vp[t]}}}
{{{xp[1]-xp[2]=-vp[2]+vp[t]}}}
{{{x(p[1]-p[2])=-vp[2]+vp[t]}}}
{{{x(p[2]-p[1])=vp[2]-vp[t]}}}  Equivalent to multiplying both sides by -1
{{{x=v(v[2]-p[t])/(p[2]-p[1])}}}
And that would be the symbolic form for x, the lower concentration material. Just substitute the given values.