A mountain-climber friend with a mass of 80 kg ponders the idea of attaching a helium-filled balloon to himself to effectively reduce his weight by 25% when he climbs. He wonders what the approximate size of such a balloon would be. Heating of your physics skills, he asks you. Share with him your calculations that show the volume of the balloon to be about 17 m3 (slightly more than 3 m in diameter for a spherical balloon).