Qian used the rule "divide the length of a piece of wood by 3 to get the number of nails needed."
If 3 nails are needed for a piece of wood that is 9 inches and 13 nails are needed for a piece of wood that is 39 inches, what is the length of a piece of wood that requires 8 nails?
We can create a proportion to solve this problem. Let x be the length of the piece of wood that requires 8 nails.
According to the given rule, we have the proportion:
9 inches / 3 nails = x inches / 8 nails
Cross multiplying, we have:
9 inches * 8 nails = 3 nails * x inches
72 = 3x
Dividing both sides by 3, we have:
24 = x
Therefore, the length of the piece of wood that requires 8 nails is 24 inches.