Question 1044390
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I think that this scheme is infinitely far from reality. 


And has nothing common with the reality.


So there is no sense to consider it and to try to solve.


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Let us consider the equation (1) and, in particular, the term 0.55z.
It is z - 0.45z.

Do you assume that the gravel consolidates/compresses "inside itself" (implodes) to zeroes its own porosity ?

Same question about the addend {{{(2/3)*y}}} for sand.

Do you know that a significant amount of water is needed and is used to produce concrete ?

Where is the water in your balance ?

The mechanism and the scheme of forming concrete is much more complicated.

The scheme in this post/solution is totally inadequate.
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