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 

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