It's called the product rule
Answer: They destroyed the bridges to make it harder for smaug to attack their town.
Explanation: Not sure just what i remember
One day, Bob set on an expedition to the mountains, where all the pterodactyls strive. He was stationed in a small camp by a cliff, with lots of grass. Since he knew that pterosaurs resist any kind if attack (assuming he needed protection) he anxiously lived out his life there.
