Question 1161169
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At the start you have:
9 qts of solution, of which 9/2 = 4.5 qts are antifreeze

You want to add x qts of antifreeze (9/2 + x  is the final amount of 
antifreeze) and have the ratio of that to the total solution (9 + x) 
equal to 60% (0.60): 

  {{{  matrix(3,3, "", "", "", ((4.5 + x)) / ((9 + x)), " = ", (0.6),""," ","") }}}

Solve for x = 9/4 = 2.25 qts  
  

Check:  (9/2 + 9/4) / (9 + 9/4) =  (27/4) / (45/4) = 27/45 = 3/5 = 0.60 (ok)