A balloon of mass m contains water of mass M. If it is completely immersed in water, the apparent mass of the balloon with water in it will be

Since, loss of weight = weight of water displaced.

Here, volume of water displaced will be equal to the volume of the balloon which is equal to the volume of the water inside the balloon. So, the mass of the water displaced = M

$$\begin{gathered} \Rightarrow \mathrm{loss} \mathrm{of} \mathrm{weight} = Mg \\ \Rightarrow \mathrm{apparent} \mathrm{weight} \mathrm{of} \mathrm{balloon} \mathrm{with} \mathrm{water} \mathrm{in} \mathrm{it} = (m+M)g-Mg=mg \\ \Rightarrow \mathrm{apparent} \mathrm{mass} \times g=mg \\ \Rightarrow \mathrm{apparent} \mathrm{mass} = m \end{gathered}$$

 So, apparent mass of the balloon when completely immersed in water will be m.