Here are the answers to the volume worksheet.  For the octagonal prism, you have to make a rt. triangle on the octagon by drawing an apothem.  The angle in the corner is 67.5 and half of the base will be 5.  Once you find the apothem, it's 1/2bh*8 to find the area where b=10 and h is the apothem.  Remember, you have to multiply the area of the octagon by the height to find volume.  Hope this helps!