A solid concrete stair has 4 steps, each with a 7-inch rise and a 12-inch run, and is 4 ft wide. Treating the cross-section as stacked steps (area = rise x run x N(N+1)/2), what is the APPROXIMATE concrete volume?

Giải thích

Rise = 7 / 12 = 0.5833 ft, run = 12 / 12 = 1 ft. Stacked-step factor N(N+1)/2 = 4 x 5 / 2 = 10. Cross-section area = 0.5833 x 1 x 10 = 5.833 sq ft. Multiply by 4 ft width: 23.33 cubic feet. Divide by 27: 23.33 / 27 = 0.864, about 0.86 cubic yards (d). Option (a) is half the correct value. Option (b) misapplies the step factor. Option (c) leaves the answer in cubic feet.