Answer:
78 inches or 174 inches (see explanation)
Step-by-step explanation:
If the perimeter is 64 feet, that means that each side is 8 feet, since there are 8 sides (64/8=8). 8 feet is 96 inches (8*12=96). 96-18=78, so the minimum length of wood he needs for the ledges is 78 inches, assuming you're not counting the wood for the perimeter, too (in which case it would be 174 inches) (78+96=174). Hope this helps!