# floating cubical block

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A cubical block of copper of side $10\ cm$ is floating containing mercury. Water is poured into the vessel so that the coper in the block just gets submerged. The height of the water column is?

Solution goes like :

Let height of water column be $h$, then $$\rho_w gh + \rho_{Hg} g(10-h)=\rho_{Cu} g 10$$

I don't understand this, why are they writing pressure using Mercury?

edited Jan 2
Mean to say, mercury will fill the space left after filling of water in the copper block?