A heat engine that does the work of blowing up a balloon at a pressure of 1 atm extracts 4 kJ from a hot reservoir at 120^oC. The volume of the balloon increases by 4 L, and heat is exhausted to a cold reservoir at a temperature Tc. If the efficiency of the heat engine is 50% of the efficiency of a Carnot engine working between the same reservoirs, find the temperature Tc.