Question 879136
You could assign variables like this:


L = 20%
H = 100%, the pure orange juice
T = 70%
M = 8
u = unknown amount of the 20%-juice drink, quarts
v = unknown amount of pure orange juice


{{{highlight_green((Lu+Hv)/M=T)}}} and {{{highlight_green(u+v=M)}}}


Refer to this lesson:
<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</a>