If $4:5 =2:x$, then find the value of $x$.

Given: $4:5 =2:x$.

To do: To find the value of $x$.

Solution: 

As given, $4:5 =2:x$

$\Rightarrow \frac{4}{5}=\frac{2}{x}$ 

$\Rightarrow 4\times x=2\times5$                            [On cross  multiplication]

$\Rightarrow 4x=10$

$\Rightarrow x=\frac{10}{4}$

$\Rightarrow x=2.5$

Therefore, $x=2.5$