70 gallons of 10% solution contain 0.1×70=7 gallons of pure antifreeze. g gallons of 50% solution contain 0.5g gallons of pure antifreeze. The volume of the mixture is 70+g, containing 0.4(70+g) gallons of pure antifreeze, so 0.5g+7=0.4(70+g)=28+0.4g. Therefore 0.1g=28-7=21, and g=21/0.1=210 gallons.