According to Mae's rule, the number of nails needed is calculated by dividing the length of the wood by 3.
Let's verify the calculations with the examples given:
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For a piece of wood that is 9 inches: \[ \text{Number of nails} = \frac{9 \text{ inches}}{3} = 3 \text{ nails} \]
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For a piece of wood that is 39 inches: \[ \text{Number of nails} = \frac{39 \text{ inches}}{3} = 13 \text{ nails} \]
Both calculations are consistent with Mae's rule.
Now, we need to find the length of wood that requires 7 nails: \[ \text{Length of wood} = \text{Number of nails} \times 3 = 7 \times 3 = 21 \text{ inches} \]
Thus, the length of wood that requires 7 nails is 21 inches.