Question 1184881
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You correctly obtained x = 14 from your equation as a solution,


but INCORRECTLY interpreted it . . . . 



These 14 gallons is the amount of the 25% solution.


So, the answer is:  14 gallons of the 25% solution, and the rest, 20-14 = 6 gallons is the amount of the 15% solution.


<U>CHECK for the concentration</U> :   {{{(0.25*14 + 0.15*6)/20}}} = 0.22 = 22%, the concentration of the final mixture.
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Solved and explained.