A system initially consists of 3 lbm of a substance having specific internal energy u of 20 Btu/lbm, and 6 lbm of the same substance having an internal energy of 30 Btu/lbm. 150 Btu of energy is transferred as heat to this system, and it is allowed to come to equilibrium. What will be the specific internal energy of the substance in the final equilibrium state ?