Question 718364
That's a good start.  Keep the variable in the case level that you first choose.  x is not X.  


You want an equation to relate the percentages of all the solutions.  
(Amount of Pure Acid)/(Amount of Solution Mixture)=(Fraction Desired).
Using the actual values given,
{{{highlight((25x+50y)/500=34)}}}.  So you system to solve is this percents equation and this one: {{{highlight(x+y=500)}}}.  Use those two as a system and solve for x and y.