Puzzling scales

In the first instance we see that a top and three cubes weigh equal to twelve marbles. In the second equation a top alone equals a cube and eight marbles. Now let us add three cubes to each side of the second scales, and as the addition of equal quantities to both sides of an equation does not change their relative values, we have the same equilibrium. By the addition of three cubes to the second pair of scales we have produced the identical values as shown by the first scales. In the first case a top and three cubes=twelve marbles; in the second illustration we have proved that a top and three cubes = four cubes and eight marbles; therefore if four cubes and eight marbles weigh the same as twelve marbles, four cubes=four marbles, so a marble weighs just as much as a cube. It proves therefore that one cube and eight marbles, or nine marbles weighs equal to the top!

9 marbles.

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