1. The problem statement, all variables and given/known data An iron casting weighs 300 N in air and 200 N in water. What is the volume of cavities in the casting, if the density of iron is 7800 kg/m3 ? 2. Relevant equations d = m/V Fg = mg 3. The attempt at a solution I was not sure in the slightest of how to solve this problem. So I tried using an approach I tried in a similar problem. Fg = mg 300 N = m * 9.8 m/s2 ==> m = 30.6 kg diron = 7800 kg/m3 dwater = 999.97 kg/m3 ddifference = -(6800.03) kg/m3 V = m/d = 30.6 kg /6800.03 kg/m3 = 0.0045 m3 I know this is probably so far from the right answer, but I literally don't have any idea where to start.