free charge density in dielectrics

Please can anyone help me to solve this problem? This problem (b) is to find E, V and C(capacitantce) in some dielectric materials. But I could’t understand the solutions about (b)

Solution manual says that the total surface charge density of (b) is +σ in upper plate. so it equals to E and ε_0V/d. But it seems wrong. +σ is not talal charge density but free charge density. isn’t it? I mean

σ_total=σ_free+σ_bound=σ + σ_bound

I wonder if I thought wrong. It would be appreciated when someone help me.

ps: I attached the problem and solution
I didn’t write in the form that PF gives, because my question is very simple.

Attached Images
File Type: png pb 4.19.PNG (49.1 KB)
File Type: jpg pb 4.19 solution.jpg (59.2 KB)

http://ift.tt/1kKNy4k

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