If x and y vary inversely and x = 18 when y = 8,

$x$ and $y$ var $y$ inversely.

i.e. $x y=$ constant

Now, $18 \times 8=x_{1} \times 16$

$\Rightarrow \frac{18 \times 8}{16}=x_{1}$

$\Rightarrow 9=x_{1}$

$\therefore$ Value of $x=9$