Question 469447
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Hi,
mixed to get 120 Liters 
Let x and (120-x) represent the amount of the  33% and 15% solutions respectively
Question states*** 
 mixing to get 120 Liters of 21% acid?
   .33x + .15(120-x) = .21*120L
Solving for x
     .18x  = .06*120L =  7.2L
      x = 40L, the amount of the 33% Solution.  80L of the 15% solution

CHECKING our Answer***
  .33*40 + .15*80 = 13.2 + 12 = 25.2 = .21*120