Proposition 21 If there are three magnitudes, and others of equal number to them, (being) also in the same ratio taken two by two, and (if) their proportion (is) perturbed, and (if), via equality, the first is greater than the third then the fourth will also be greater than the sixth. And if (the first is) equal (to the third then the foruth will also be) equal (to the sixth). And if (the first is) less (than the third then the fourth will also be) less (than the sixth).