A 3 in horizontal drain is run 72 ft at 1/8 in per ft. The invert at the upstream end is at elevation 100.00 ft. What is the invert elevation at the downstream end?

Giải thích

The fall is 72 ft x 1/8 in = 9 in, and 9 in equals 0.75 ft, so the downstream invert is 100.00 - 0.75 = 99.25 ft. The 99.85 ft and 99.90 ft answers understate the drop by treating the fall as inches of elevation rather than converting to feet, and 98.50 ft doubles the correct fall.