Three portions of the same liquid are mixed in a container that prevents the exchange of heat with the environment. Portion A has a mass m and a temperature of 99.0°C, portion B also has a mass m but a temperature of 78.6°C, and portion C has a mass mC and a temperature of 36.0°C. What must be the mass of portion C so that the final temperature Tf of the three-portion mixture is Tf = 46.0°C? Express your answer in terms of m.