How many cubic meters of helium are required to lift a balloon with a 400-kg payload to a height of 8 000 m? (Take PHe = 0.180 kg/m3.) Assume that the balloon maintains a constant volume and that the density of air decreases with the altitude z according to the expression pair = p0e-z/8 000, where z is in meters and p0 = 1.25 kg/m3 is the density of air at sea level.