A spherical balloon of 1.4 m diameter is filled with helium. The mass of the balloon skin is 0.1 kg. If at a certain altitude the density of air is 1.0 kg/m^3 and is six times the density of helium in the balloon, determine the steady upward speed of the balloon in m/s. The viscosity of air is 1.8x10^-5 Pas and the acceleration of gravity is 9.8 m/s^2. Assume the drag coefficient of the balloon to be 0.41.