Liquid helium is stored at its boiling-point temperature of 4.2 Kin a spherical container (r=0.30 meter). The container is a perfect blackbody radiator. The container is surrounded by a spherical shield whose temperature is 67 K. A vacuum exists in the space between the shield and the container. The latent heat of vaporization for helium is 2.1x104 J/kg. Find what mass of liquid helium boils away through a venting valve in 1 hour?