A detail drawing shows a foundation wall with a scale of 3/4" = 1'-0". The wall thickness measures 1.5 inches on the drawing. What is the actual wall thickness?
Correct Answer
D) 24 inches
With a scale of 3/4" = 1'-0", each 3/4 inch on the drawing represents 12 inches actual. Therefore, 1.5 inches on the drawing equals 24 inches actual (1.5 ÷ 0.75 × 12 = 24).
Why This Is the Correct Answer
Option C is correct because when using the scale 3/4" = 1'-0", we need to determine how many scale units fit into the measured drawing dimension, then multiply by the actual dimension each unit represents. Since 1.5 inches ÷ 0.75 inches = 2 scale units, and each unit represents 12 inches actual, the result is 2 × 12 = 24 inches. This calculation properly converts the drawing measurement to real-world dimensions using the given architectural scale.
Why the Other Options Are Wrong
Option B: 12 inches
12 inches is incorrect because this would only be true if the drawing measurement was 0.75 inches (exactly one scale unit). The calculation would be 0.75 ÷ 0.75 × 12 = 12 inches, but our drawing measurement is 1.5 inches, which is twice that amount.
Option C: 30 inches
30 inches is incorrect and likely results from calculation errors in the scale conversion process. This might occur from incorrectly setting up the proportion or making arithmetic mistakes in the division and multiplication steps.
Memory Technique
Remember 'DDA': Divide drawing measurement by scale Drawing unit, then multiply by Actual unit. For 3/4 scale, think '3/4 gets you 12' - so figure out how many 3/4 units you have, then multiply by 12.
Reference Hint
Florida Building Code Chapter 1 - Scope and Administration, or architectural drafting standards section covering scales and drawing interpretation
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