Typical hot air balloons used on sightseeing flights attain volumes of 3000 m3. A typical gross weight (balloon, basket, fuel and passengers, but not the air in the balloon) on such a balloon flight is 600 kg. If the ground temperature is 20?C, the lapse rate is zero, and the balloon is in hydrostatic equilibrium at a cruising altitude of 900 hPa, determine the temperature of the air inside the balloon.