When two material with different concentrations of the same active ingredient are mixed, the active ingredient before and after mixing remains constant. If the volume of the first brand is assigned x gallons, the volume of the second brand is (160-x) gallons. Therefore, according to the above principle, (65%)(x) + (90%)(160-x) = (70%)(160), 0.65x+ 144-0.9x = 112, x=128 gallon. 128 gallon of the first brand (65%) and 160-128 = 32 gallon of the second brand (90%) of antifreeze must be used.
