Says A to B: "Give me four weights and I can weigh any number of pounds not exceeding 40." Required the weights and the method of weighing.
Answer. The weights are 1, 3, 9, and 27 pounds. In weighing we must put one or more in both scales, or some in one scale and some in another: thus, 7 lbs. = 9 lbs. + 1 lb. - 3 lbs.