Two solid spheres A and B of equal volumes but of different densities dA and dB are connected by a string. They are fully immersed in a fluid of density dF. They get arranged into an equilibrium state as show in the figure with a tension in the string. The arrangement is possible only if :

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system will be in equilibrium with tension in string only if df > dA and dB > df. If both A & B are considered as a system then
2Vdfg = V (dA + dB)g dA + dB = 2df