We have solutions of 20% acid and 40% acid. We need 60 liters of 40% acid. "t" is liters of 20% and "f" is liters of 40% We have to set up two equations: A) t + f = 60 B) .20t + 1.0f = (60*.40) Multiplying equation A) by -.2 A) -.2t -.2f = -12 then adding that to B) B) .2t + f = (24) .8f = 12 forty per cent volume = 15 liters A) t + f = 60 Twenty per cent volume = 45 liters Source: http://www.1728.org/mixture.htm