Given three numbers, to investigate when it is possible to find a fourth proportional to them.

Let *A, B,* and *C* be the given three numbers. It is required to investigate when it is possible to find a fourth proportional to them.
[The Greek text of this Proposition is corrupt. However, analogously to Proposition 18 the condition that a fourth proportional to *A, B,* and *C* exists is that *A* measure the product of *B* and *C.* ]

Q.E.D.