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Everything is clear to me in this post, except one thing: does the box have the upper lid or not ?
It seems that the problem must define it - but is does not.
From the other side, the problem does not mention about the top or bottom surfaces and says nothing about the base and so on . . .
Based on it, I will assume that the wooden material is at each of 6 faces of the box.
Exterior dimensions of the block are 6, 5 and 8 inches.
The volume of the box, calculated via exterior dimensions is 6*5*8 = 240 in^3.
Interior dimensions are 6-2 = 4, 5-2 = 3 and 8-2 = 6 inches.
The volume of the empty space inside the block, calculated via interior dimensions is 4*3*6 = 72 in^3.
The volume of the material used = 240 - 72 = 168 in^3.
How many wooden blocks . . . = 168.
I answered all questions.