Question

Find the fourth proportional of:​ (iv) $\frac{1}{3},\frac{2}{5}$ and $8.4$.

Answer 1

Let the fourth proportional be $x$.
$\therefore$ $\frac{1}{3}:\frac{2}{5}=8.4:x$
[$\because$ Ratio of first two terms $=$ ratio of last two terms]

$\Rightarrow$ $\frac{1}{3}$$÷$$\frac{2}{5}=\frac{8.4}{x}$
$\Rightarrow$ $\frac{1}{3}$$\times$$\frac{5}{2}=\frac{8.4}{x}$
$\Rightarrow$ $\frac{5}{6}=\frac{8.4}{x}$
$\Rightarrow$ $x=\frac{6\times 8.4}{5}=\frac{50.4}{5}=10.08$
Hence, the fourth proportion is $10.08$.