$6-137$ Consider two Carnot heat engines operating in series. The first engine receives heat from the reservoir at $1400$ K and rejects thewaste heat to another reservoir at temperature T$.$ The second engine receives this energy rejected by the first one, converts some of it towork, and rejects the rest to a reservoir at $300$ K$.$ If the thermal efficiencies of both engines are the same, determine the temperature TAnswer: $648$ K