First, we will determine the actual dimensions of the garden using the given scale. The scale states that 1 inch on the blueprint corresponds to 4 feet in reality.
The garden's dimensions from the blueprint are:
- Width: 3 inches
- Length: 5 inches
Now, we will convert these measurements to actual feet:
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Width in feet: \( 3 \text{ inches} \times 4 \text{ feet/inch} = 12 \text{ feet} \)
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Length in feet: \( 5 \text{ inches} \times 4 \text{ feet/inch} = 20 \text{ feet} \)
Next, the problem states that the actual garden is doubled in size. To determine the new dimensions, we will double both the width and the length:
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New Width: \( 12 \text{ feet} \times 2 = 24 \text{ feet} \)
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New Length: \( 20 \text{ feet} \times 2 = 40 \text{ feet} \)
The question specifically asks for the new length of the garden after it is doubled in size.
Thus, the new length of the garden is: \[ \boxed{40} \text{ feet} \]