Solving Consolidation Pressure: Half Layer of Clay?

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Homework Statement

I don't understand why the pressure before the construction of foundation is given by (2.5)(16.5) + (0.5)(17.5-9.81) +(1.25)(16-9.81) only ?. Why the pressure is taken only to the half of the layer of clay only ?

Homework Equations

The Attempt at a Solution

I think it's wrong , it should be (2.5)(16.5) + (0.5)(17.5-9.81) +[/B](2.5)(16-9.81) ..Correct me if I am wrong

 

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Anyone has answer ? I seriously think it's wrong. Can anyone guide me ? Thanks in advance !