You just need to know the formula for the volume of a 
retangular solid (like a box or brick).

Volume = width × length × height

Here you have to interpret "thickness" as "height", so for this
problem, its:

Volume = width × length × thickness 

Volume = 1,107,096 cubic inches

Width = 13ft, 7inches = (13×12 + 7) inches = (156+7) = 163 inches

Length = 27ft, 7inches = (27×12 + 7) inches = (324+7) = 331 inches

We substitute those into the formula:

Volume = width × length × thickness

1107096 = 163 × 331 × thickness

1107096 = 53953 × thickness

So we divide 1107096 by 53953 and get 20.51963746 inches.

But nothing is measured that accurately, so we round off
to the estimation 20.5 inches.


Edwin