Two rigid boxes containing different ideal gases are placed on a table. Box A contains one mole of nitrogen at temperature ${T_0}, while Box contains one mole of helium at temperature \left( {{7 \over 3}} \right){T_0}. The boxes are then put into thermal contact with each other, and heat flows between them until the gases reach a common final temperature (ignore the heat capacity of boxes). Then, the final temperature of the gases, {T_f} in terms of {T_0}$ is