If four number are proportional then the number created from (multiplying) the first and fourth will be equal to the number created from (multiplying) the second and third. And if the number created from (multiplying) the first and fourth is equal to the (number created) from (multiplying) the second and third then the four numbers will be proportional.
In modern notation, this proposition reads that if \[\frac ab=\frac cd,\] then \[a\,d=b\,c,\] and vice versa, where all symbols denote numbers.
Proofs: 1