A number $x$ completes the proportion 8 : 14 :: 20 : $x$. This $x$ is known as the fourth proportional. What is its value?

If a : $b=c:d$, the four numbers are in proportion. The classic check: the **product of the extremes** $(a\times d)$ equals the **product of the means** $(b\times c)$. Example: are 4 : 6 and 6 : 9 in proportion? Cross-multiply: $4\times 9=36,6\times 6=36$. Equal → yes, in proportion.