In a hostel of 50 girls, there are food provisions for 40 days.

Let $x$ be the number of days with food provisions for 80 (i.e., $50+30$ ) girls.

Since the number of girls and the number of days with food provisions are in inverse variation, we have:

$50 \times 40=80 x$

$\Rightarrow x=\frac{50 \times 40}{80}$

$=\frac{2000}{80}$

$=25$

Thus, the required number of days is 25 .