Principle of Carius method
In this method, chlorine present in the organic compound is converted into $\mathrm{AgCl}$.

Mass relation in $\mathrm{AgCl}$
Molar mass of $\mathrm{AgCl}$: $108 + 35.5 = 143.5\,\text{g mol}^{-1}$

So, in $143.5$ g of $\mathrm{AgCl}$, mass of chlorine $= 35.5$ g.

Therefore, in $143.5$ mg of $\mathrm{AgCl}$, mass of chlorine is: $\frac{35.5}{143.5} \times 143.5 = 35.5\,\text{mg}$

Percentage of chlorine in the compound
Mass of organic compound $= 180$ mg.

Mass of chlorine in it $= 35.5$ mg.

Hence, $\%\,\text{Cl} = \frac{35.5}{180} \times 100$ $= 19.72\%$

Integer answer
$\boxed{20}$

Comparison with stored answer
Stored correct answer is $20$, which matches the computed integer value.