Question 621059
Those "5 steps for problem solving" are a mystery to me.
I solve problems my own way, using my own reasoning and not a pre-set recipe.
I believe that using recipes is the wrong way to teach math.
 
The volume of a rectangular right prism can be calculated as
Volume = length X width X height
If the volume (V, in cubic feet) has to be at least 72 cubic feet, and the height (or thickness) of the slab is x feet,
{{{V>=72}}} and {{{V=12*8*x}}} can be combined to get
{{{12*8*x>=72}}} --> {{{x>=72/8/12}}} --> {{{x>=3/4}}}
So the concrete slab should be at least 3/4 of a foot (9 inches) thick.