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