A girl has stolen two bars of solid gold, each weighing 18 pounds, and is running away from police. On the path she is on, she comes up to a rope bridge with a sign saying that the rope bridge can hold a maximum weight of 130 pounds, and that if any more weight is placed on the bridge it will collapse.
The girl knows her weight is 100 pounds, so she walks across the bridge carrying both gold bars, and the bridge does not break. How is this possible?
Gold is weighed in troy ounces or troy pounds, where there are only 12 troy ounces per troy pound. The maximum weight specified on the bridge will be using the normal imperial system for measuring weight, where there are 16 pounds to an ounce.
Because of this 36 pounds of gold (weighed using the Troy system), will only weigh about 29 standard imperial pounds. This means that her total weight, including both gold bars, is less than 130 pounds.
An alternative answer would be: The girl would simply throw one of the gold bars up in the air while holding the other in a juggling manner. Thus, she would never be holding more than 118 pounds as long as she threw the opposite bar up before she caught the other.