Solution: The number of intervals is $ rac181.5 = 12 $. Since beams are placed at both ends, total beams = $ 12 + 1 = 13 $. However, the problem states 4 beams per 3 meters, which implies $ rac43 $ beams per meter. For 18 meters: $ rac43 imes 18 = 24 $. oxed24