Physics
posted by Sierra .
You want to design a heliumfilled balloon that will lift a total payload of 1225 kg. What volume of helium is needed to just barely lift this payload?

Use Archimedes' Principle.
Volume*(Air density) = Payload + (Volume)*(Helium density)
Solve for Volume.
You will need to assume a temperature and pressure. I suggest 15 C and 1 atmosphere.
At sea level and at 15°C , the density of air is 1.275 kg/m^3. Helium would be 4/29 of that, or 0.176 kg/m^3
V = 1225 kg/(1.275  0.176)kg/m^3
= 1115 m^3
The "payload" is assumed to include the weight of all balloon materials, not just cargo