To find the scale factor used in Ian's drawing, we begin by converting the actual length of the desk from feet to inches since the scale drawing is measured in inches.
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Convert the actual desk length to inches: \[ 6 \text{ feet} = 6 \times 12 \text{ inches} = 72 \text{ inches} \]
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Determine the scale factor: The scale factor is the ratio of the length in the drawing to the actual length: \[ \text{Scale Factor} = \frac{\text{Length in Drawing}}{\text{Actual Length}} = \frac{3 \text{ inches}}{72 \text{ inches}} \]
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Simplify the fraction: \[ \frac{3}{72} = \frac{1}{24} \]
Thus, the scale factor that Ian used in the drawing is: \[ \boxed{\frac{1}{24}} \]