You plan to paint the outside of this garage, including the roof. Which calculation will help you best decide how much paint you will need? (Omit the floor of the garage in your calculation.) Add the areas of 9 rectangles and 2 triangles. Add the areas of 6 rectangles and 2 triangles. Add the volumes of 1 rectangular prism and 1 triangular prism. Add the perimeters of 4 rectangles and 2 triangles.

The four walls will be four rectangles.
The roof will be two rectangles.
The ends holding up the roof will be two triangles.

The 2nd selection is appropriate (areas of 6 rectangles + 2 triangles).

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Yes, you're trying to find the volume of paint, but paint is sold in terms of the number of square feet (area) it covers. Rarely do we concern ourselves with computing paint volume from the thickness of the film covering the area. Certainly, the volume of paint is substantially less than the volume of the walls or the entire garage.