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Cut The Knot!
by Alex Bogomolny
The Three Jugs Problem
Two friends who have an eight-quart jug of water wish to share it evenly. They also have two empty jars, one holding five quarts, the other three. How can they each measure exactly 4 quarts of water?
In [Ball & Coxeter, p. 27] the problem appears with the remark that "the solution presents no difficulty." It precedes another problem with 4 jugs of capacities 5, 11, 13, and 24 for which a solution "can be worked out only by trial." The problem is presented by the narrative:
Three men robbed a gentleman of a vase, containing 24 ounces of balsam. Whilst running away they met a glass seller, of whom they purchased three vessels. On reaching a place of safety they wished to divide the booty, but found that their vessels could hold 5, 11, and 13 ounces respectively. How could they divide the balsam into equal portions?
If nothing else, such problems wrap up a meaningful counting exercise that can be handed out to children in early grades.
But there is also some worthwhile mathematics involved that was mostly overlooked by teachers and students alike. (This is judging by the number of questions I received concerning existence of a solution.)
Label the jugs A, B, C in the increasing order of their capacities. Let's agree to use the same letters for the capacities themselves. Let x, y, z denote the quantities of water in the jugs. In particular,
Here's the statement of existence:
Let C = A + B, where A and B are mutually prime. Then any quantity
Start with (0, 0, C), and pour from C to A and from A to B to obtain
The problem and the proof have a surprising geometric interpretation [Coxeter & Greitzer, p. 89] in terms of triangular coordinates. Cartesian and polar coordinates are great tools of analytic geometry of the plane. Spherical coordinates are suitable for the geometry of the sphere, as are cylindrical coordinates on a cylinder. There are several systems of coordinates in which vertices and sides of a triangle are treated in an equitable manner. The most important are the barycentric and trilinear coordinates.
For a point P in the plane of ΔABC, the triple of its (signed) distances to the sides BC, CA, and AB is called trilinear coordinates of P (with respect to ΔABC.) The distances are signed such that, for example, the distance to AB is positive or negative depending on whether P is located on the same or different side of AB as vertex C. The barycentric coordinates are defined as a triple of (signed) areas of triangles APB, BPC, and CPA. In both systems, location of P is fully defined by any two numbers; the third number is redundant. For this reason, the coordinates are considered as homogeneous.
For example, if r is the inradius - the radius of the inscribed circle - of ΔABC, then trilinear coordinates of its incenter are
The Description of the 3 Jugs problem as a triple of quantities
The (integer) points that satisfy built-in constraints of the 3 Jugs problem (e.g.,
The jug B is full at the points of the "western" side of the parallelogram. Close to that side, the left leg of the inverted V may not reach the bottom line of the parallelogram. In which case, a secondary move must be made: first parallel to the line BC to the "eastern" side of the paralellogram (pour B to C), and then to the bottom side (pour from A to B.)
Since we are only interested in the modular arithmetic, we may overlook the need for secondary moves on the western side of the parallelogram and keep applying only the basic moves. Since A and C are mutually prime, all lines
Condition A + B = C serves a double purpose. First, together with the relative primality of A and B, it insures that all three capacities share no common factor, save 1. Were they not, the quantities that could be measured with three vessels of the specified capacities would share their common factor. For the problem to be generally solvable the mutual primality of all three capacities is a necessary condition. However, it is not sufficient. Anomalies also arise when the three jugs are rather big. Condition
This completes the analysis of existence of a solution to the 3 Jugs problem. That of the 2 and 4 Jugs problems is left as an enticement for the future Poissons.
- W. W. Rouse Ball and H. S. M. Coxeter, Mathematical Recreations and Essays, Dover, 1987
- H. S. M. Coxeter and S. L. Greitzer, Geometry Revisited, MAA 1967
- E. Kasner and J. Newman, Mathematics and the Imagination, Simon and Schuster, 1958
- The Three Jugs Problem. Introduction and a story
- 3 Glasses Puzzle
- Water puzzle, experimental math
- Three Glass Puzzle (Graph Theoretical Approach)
- The puzzle in barycentric coordinates
- Two Pails Puzzle
- Plain Gadgets
- 3 Jugs Problem - A Water Doubling Variant