To weigh a 40-pound object with four weights – 1, 3, 9, and 27 pounds, the merchant placed all of the weights on one side of the scale and the object on the other side. But how did he weigh 5 pounds?
Answer
He would need to place the 3-pound weight and the 1-pound weight on the same side as the object and balance it with the 9-pound weight. If the 2 groups weigh the same, then the weight of the object is 5 pounds.
When you place a weight on the same pan of the scale as the object you are weighing, you subtract that weight from the total of the weights on the pan on the other side.