Question 892549
Putting directly into initial formula equations can be this way:


Let u = how much of the a%
Let v = how much of b%


{{{highlight_green((au+bv)/x=c)}}} and {{{highlight_green(u+b=x)}}};
Solve the system for u and v.


Study this as a lesson which corresponds to the question you asked:
<a href="http://www.algebra.com/tutors/Mixtures%3A-All-in-Symbols.lesson?content_action=show_dev">Mixing two concentrations for known amount of mixture - http://www.algebra.com/tutors/Mixtures%3A-All-in-Symbols.lesson?content_action=show_dev</a>