This was an extra question in an exam I took today. The temperature of the air in the bottle remains constant. I know this has something to do with buoyancy and ideal gases since temperature was mentioned in the question, and I think I could have easily solved the question, had the submerged object been a solid. The bottle starts filling up with water as the hydrostatic pressure increases as the bottle is pushed deeper and deeper into water, so the "average density" of the contents of the bottle start approaching the density of water, as the ratio of water to air increases inside the bottle. The formula for hydrostatic pressure ph=(ro)*g*h has the required depth of water in it. The pressure of air increases as the volume of the air pocket in the bottle decreases. This much I understand. However this is as far as I got. The mass of the bottle was something like 0,380kg; and it's volume was 0,55 liters (= 0,55dm3 = 0,00055m3). What information do I take advantage of to solve this type of question? My brain is jamming up right now.