Sonal attended her school on 204 days in a full year.

Let $x$ be the number of days the school was opened.

Number of days Sonal attended school $=204$ days

Percentage of her attendance $=85 \%$ of $\mathrm{x}$

$=\left(\mathrm{x} \times \frac{85}{100}\right)$

$=\frac{85 \mathrm{x}}{100}$

Now, $\frac{85 x}{100}=204$

$\Rightarrow x=\left(204 \times \frac{100}{85}\right)$

$\Rightarrow x=240$

$\therefore$ The school was opened for 240 day.