\(x\) is said to be the fourth proportional of the numbers \(a, b\) and \(c\) if 

\(a : b :: c : x\) \( \Rightarrow \) \(\frac{a}{b} = \frac{c}{x}\) or \(x =\) \(\frac{{bc}}{a}\) 

Example 8: Find the third proportional to 15 and 20.

Solution: Suppose \(x\) is the third proportional to 15 and 20. Then 

\(15:20::20:x\) \( \Rightarrow \) \(\frac{{15}}{{20}} = \frac{{20}}{x}\) or \(x\) =\(\frac{{20 \times 20}}{{15}}\)=\(\frac{{80}}{3}\)