Question 393122
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Hi
Let x represent  the total amount of the 90% solution
Question states***
Mixing to form (100+x) gallons of 75%
 .70*100gal + .90x = .75(100gal+x)
solving for x
          .15x = .05*100gal
             x = .05*100gal/.15
             x = 33 1/3 gal