8 gallons of 55% acid solution contain 0.55*8 gallons of the pure acid. 12 gallons of 65% acid solution contain 0.65*12 gallons of the pure acid. The mixture contains 0.55*8 + 0.65*12 gallons of the pure acid and has the volume of 8 + 12 = 20 gallons. Therefore, the concentration under the question ispercents. Notice that I multiplied by 100 to get the percentage instead of fraction. You complete all necessary calculations.