If a, b, c be quantities of one, and d, e, f magnitudes of the same or any other kind, such that

a : b : c = d : e : f,

and if

a > c, then d > f,

but if

a = c, then d = f,

and if

a < c, then d < f.

Prop. 21. If

a : b = e : f and b : c = d : e,