Archimedes principle. The weight of the bouy is given, so the weight of water displaced by the submerged part of the bouy equals that. Figure the volume that weight of water displaces, then figure the height of a .33m diameter column of water will have of that volume.

A large buoy used to support an oceanographic research instrument is made of a cylindrical, hollow iron tank. The tank is 2.1 m in height and 0.33 m in diameter. The total mass of the buoy and the research instrument is about 120 kg. The buoy must float so that one end is above the water to support a radio transmitter. Assuming that the mass of the buoy is evenly distributed, how much of the buoy will be above the waterline when it is floating?

I'm not sure what to do or which equations to use...Please help! Thanks!