A site plan shows contour lines at 2-foot intervals. If you count 6 contour lines between two points that are 300 feet apart horizontally, what is the slope percentage between these points?
Correct Answer
B) 4%
Six 2-foot interval contour lines represent 12 feet of vertical change (6 x 2 = 12). Slope percentage is rise over run times 100: (12 ÷ 300) x 100 = 4%.
Why This Is the Correct Answer
Option B is correct because slope percentage is calculated as rise over run multiplied by 100. The vertical rise is 6 contour lines × 2 feet per line = 12 feet. The horizontal run is given as 300 feet. Therefore, the slope percentage is (12 ÷ 300) × 100 = 4%.
Why the Other Options Are Wrong
Option A: 2%
This incorrectly uses only the contour interval (2 feet) without accounting for the total number of contour lines crossed, resulting in an incomplete calculation.
Option C: 6%
This incorrectly uses just the number of contour lines (6) as the percentage without properly calculating the rise over run formula.
Option D: 12%
This appears to multiply the rise (12 feet) by 100 and divide by 100, or confuses the calculation by not properly applying the rise over run formula.
Memory Technique
Remember 'RRH': Rise over Run times Hundred. Count contour crossings, multiply by interval for rise, then divide by horizontal distance and multiply by 100.
Reference Hint
Look up site work and grading sections in construction reference materials, typically found in earthwork or site preparation chapters
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