We need 5 liters of 27.8% acid. A) x + y = 5 B) .50x + .13y = (.278 * 5) We multiply A by -.5 A) -.5 x + -.5 y = -2.5 then we add it to B B) .50x + .13y = (<span>1.39 </span>) -.37y = -1.11 y = 3 x = 2 So, we need 2 liters of 50% and 3 liters of 13%.