Acylindrical piece of iron pipe is shown below. the wall of the pipe is 0.75 inch thick, and the pipe is open at both ends:

what is the approximate inside volume of the pipe?

Volume of a cylinder can be defined as:  V = πr²h

h = 12 in, but to find "r" we have to consider the thickness of the wall of the pipe.  Right now the dimension given for the diameter is 8in.  We need to actually subtract the thickness of the pipe wall twice to obtain the diameter of the interior portion of the pipe.. So 8 - 1.5 = 6.5 or the diameter we can use to find the radius we will work with... 6.5 divided by 2 = 3.25 
r = 3.25
Now lets substitute into our formula: V = π(3.25)²(12)
Your volume is 398.2 in² rounded to the nearest tenth