An operator is grading a subgrade for a building foundation with a target slope of 0.5% for positive drainage. If the design elevation at the high point is 100.000 metres, what should the elevation be 25 metres horizontally away, down the slope?

Question: An operator is grading a subgrade for a building foundation with a target slope of 0.5% for positive drainage. If the design elevation at the high point is 100.000 metres, what should the elevation be 25 metres horizontally away, down the slope?

Answer options:

• 99.875 metres ✅ 99.925 metres
• 100.075 metres
• 100.125 metres

Correct answer: 99.925 metres

Explanation: A 0.5% slope means a drop of 0.5 metres per 100 metres horizontal distance. For 25 metres, the drop would be (0.5 / 100) * 25 = 0.125 metres. So, 100.000 metres - 0.125 metres = 99.875 metres. My calculation is incorrect. 0.5% means 0.005. So 0.005 x 25m = 0.125m drop. 100.000 - 0.125 = 99.875m. Let's recheck options. Okay, 99.875 is an option. If it's a 'positive drainage' slope, it's typically down, so subtract. My calculation was correct, but I selected the wrong option index. Let's redo: target slope of 0.5%, meaning 0.5 units vertical for every 100 units horizontal. For a horizontal distance of 25 metres, the vertical change is (0.5 / 100) * 25 = 0.125 metres. Since it's 'down the slope', subtract this from the high point: 100.000 m - 0.125 m = 99.875 m.