Surface in capacitor equation

i want to calculate the capacitance of a two-surface parallel capacitor with famous equation

as you see the attachment (question 28) writer divided the depth size by 3. but if we know the surfaces of the capacitor are rectangular such as this picture

and each surface of capacitor has depth=15cm and width=20cm
so S=.15*.20=.03
but why the writer divided the depth size by 3(15/3)?because their depth are same and width should be changed via the figure.

Attached Files
File Type: pdf ch6.pdf (237.0 KB)

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