Buoyancy force
= Volume*(air density - helium density)- (balloon fabric mass)
= desired mass reduction = 20 kg
Solve for the volume.
You will have to assume a gas pressure and temperature. You should also take into account the mass of the material of the ballon, but they probably expect you to ignore it.
A mountain climber friend with a mass of 80kg 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 baloon would be.
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