1. The problem statement, all variables and given/known data A girl of mass 40 kg and volume of 40 liter wants to keep her head of volume 3 liter above water surface as she can't swim . how much air in Kg should she fill in her jacket?? 2. Relevant equations 3. The attempt at a solution mass of girl (m) = 40 kg volume of girl (V) = 0.04 m^3(meter cube) density of girl (D) = m / V = 1000 kg/m^3 volume of head (v') =0.003 m^3 let the mass of air in kg be x then total mass of body = 40 + x mass of water displaced = 40 + x volume of water displaced = 1000(40 + x) then volume of immersion = V - v' = .04- .003 = .397 therefore from principle of flotation volume of water displaced = volume of immersion of floating body 40000 + 1000x = .397 x=.397-40000 = -ve value is my method correct??? and as " density of body = density of water " will require extra air to float.