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Real depth / apparent depth = refractive index

   h1 / 6 cm = μ    =>  h1 = 6 μ  cm

   h2 / 4cm =  μ    =>  h2 = 4 μ  cm

        h1 + h2 =  15 cm = 10 μ    =>    μ = 1.5 cm

The bubble is in the middle of the width of the cube.  So real depth from h1 one side face is equal to (15-h1) of the real depth from the opposite face.

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