1. The problem statement, all variables and given/known data A 1.5 m3 tank containing air at 35°C and 750 kPa is connected by a valve to another tank containing 8.25 kg of air at 65°C and 265 kPa. Now the valve is opened and the entire system is allowed to reach thermal equilibrium with the surroundings, which is at 25°C. Determine the final equilibrium temperature. 2. Relevant equations I know that for simple thermal equilibrium problems involving only two substances, the equation I would use is [mcΔT]1 + [mcΔT]2 = 0. I've read that when 3 substances are in thermal equilibrium with each other, I would simply need to expand on the previous equation and make it [mcΔT]1 + [mcΔT]2 + [mcΔT]3 = 0, but since I'm not given the mass of the surroundings, and I don't know enough to figure that out, I can't use this equation. Because of this, I'm not sure what equation to use. 3. The attempt at a solution Since I'm not sure what equation to use, I can't attempt to find the solution. If someone could please help me, I would really appreciate it. Thanks.