Question 704186
This same question is asked many times in many ways for so many situations.

x amount of low concentration material = unknown
L concentration of low conce. material = 0.46
y amount of higher concentration material = unknown
H concentration of higher conc. material =0.67
T Target concentration wanted in the mixture = 0.55
a amount of target concentration mixture =42 liter


Your system of equations to solve:
{{{x+y=a liters}}}

{{{(x*L+y*H)/a=T}}}


Start from the system by multiplication of the T equation by a.
{{{x*L+y*H=a*T}}}
Use the liters equation to substitute for either x or y in the adjusted T equation, and continue...