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**HallsofIvy** Are we to assume, since 5.7% of the phosphoric acid is retained, that, in fact, 5.7% of the original liquid is retained? If not, I see no way to do this. If so, since you know there were 45kg of liquid, there must be .057(45)= 25.65 kg of the original liquid in the second cake. You know the mass of solids in the second cake and their percent so you can calculate the mass of the second cake. You know the percentage of the second cake that is water so you can calculate the mass of the water in the cake and, subtracting of the mass of water retained, calculate the mass of "new" water in the cake.