The U.S. energy demand is estimated to be 3600 GW · yr of energy each year by 2015. Estimate the length of time that the U.S. coal reserves of 2.5 × 1015 kg will last if they are used exclusively to provide 100 percent of the U.S. energy demand at a thermal efficiency of 35 percent. Assume an average heating value of 26,600 kJ/kg for coal.