A closed container contains a homogeneous mixture of two moles of an ideal monatomic gas ( $\gamma=5 / 3$ ) and one mole of an ideal diatomic gas ( $\gamma=7 / 5$ ). Here, $\gamma$ is the ratio of the specific heats at constant pressure and constant volume of an ideal gas. The gas mixture does a work of 66 Joule when heated at constant pressure. The change in its internal energy is $\_\_\_\_$ Joule.