Question

Find the fourth proportional in $3,4,15$

Answer 1

Let the fourth proportional be $\text{x}$,

Then, the numbers becomes in proportion.

$\text{⇒3:4::15:x}$

Therefore,$\text{Product of extremes = Product of means}$

$$\begin{gathered} \text{⇒x×3=4×15} \\ \text{⇒3x=60} \\ \text{⇒x=}\frac{60}{3} \\ \Rightarrow \mathrm{x}=20 \end{gathered}$$

Hence, the fourth proportional is $20$