Question

Find the third proportion of 16 and 8.

Answer 1

We have to find c such that $16:8=8:c$. Thus
$c=\frac{8^2}{16}=\frac{64}{16}=4$.
Hence the third proportion of $16$ and $8$ is $4$.
In a proportion $a:b=c:d$, we say d is the fourth proportion of $a,b,c$.