Proportion

$\text{When two variables x and y, are inversely proportional, then: }$

$y=\dfrac{k}{x}$  or $x=\dfrac{k}{y}$ where k is a constant to be determined.

So, let us use: $y=\dfrac{k}{x}$

How to find k? Use the given:  $x=2,y=4$

$4=\dfrac{k}{2} \implies k=8$

Thus, $y=\dfrac{8}{x}$

Now, when y=1 then: $1=\dfrac{8}{x} \implies x=8$

Hope this helps!