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The volume fraction is 30-21√2, which is approximately 0,30.
Setting the cube side to 1 for convenience, we have to find the pyramid side a and height h. We can extract this from the cross section shown above. From the Pythagorean theorem in the left side, we derive h=a/√2.
Finally, the cube’s volume is 1 and the pyramid’s is a2h/3, leading to the desired result.