An explosion breaks a rock into three parts in a horizontal plane. Two of them go off at right angles to each other. The first part of mass \(1\mathrm{kg}\) moves with a speed of \(12\mathrm{ms}^{-1}\) and the second part of mass \(2\mathrm{kg}\) moves with speed \(8\mathrm{ms}^{-1}\) . If the third part flies off with speed \(4\mathrm{ms}^{-1}\) then its mass is