Plane Geometry

Hello.. and good evening.

I would like to know how to solve for the area of the mid-section of this prismatoid. I tried solving for the volume and I got an answer of 375 cu. ft. but according to Kern and Bland's book the real answer is 310 cu.ft. I guess I made a mistake in solving it.

Can I know the complete solution and the illustration of the top view of the prismatoid's mid-section? Thank you very much to whoever will answer my question. It would be a great help for me.

1)An excavation is 12 ft. deep and has trapezoidal sides (faces). The upper base is horizontal rectangle 400 ft. by 180 ft., and the lower base is a horizontal rectangle 350 ft. by 150 ft. How many cubic yards of earth were removed in digging the excavation?

2)A chimney in the shape of a frustum of a regular pyramid is 186.3 ft. high. Its upper base is a square 10 ft. on a side, and its lower base is a square 16 ft. on a side. The flue is of uniform square cross section. 7 ¼ by 7 ¼ ft. find the weight of the chimney if the material weighs 112.8 lb. per cu. ft.

How can i compute the height of the ice cube to be put in a conical tank when it turns into a liquid (water). Given with a height of the conical tank and its radius and the volume of the cube.