Jessica's pumpkin weighs half the weight of Brad's 54-pound pumpkin so 54/2 = <<54/2=27>>27 pounds
Betty's pumpkin weighs 4 times the amount of Jessica's 27-pound pumpkin so it weighs 4*27 = <<4*27=108>>108
Betty has the heaviest pumpkin at 108 pounds while Jessica's is the lowest at 27 pounds so the difference is 108-27 =<<108-27=81>>81 pounds
#### 81