Question 410648
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Hi
to obtain 40 gallons 
Let x and (40gal-x) represent the amount of the 20% and 5% solutions respectively
Question states***
 .20x + .05(40gal-x) = .12*40gal
Solving for x
     .15x = .07*40gal
        x = .07*40gal/.15
        x = 18 2/3 gal

CHECKING our Answer***
 .20*18.667 + .05*21.333 = 4.8 = .12*40