In a hostel of 50 girls, food is available = 40 days
For 1 girl, food provision = 50× 40 = 2000 days
Now, for (50 + 30) girls i.e. 80 girls, the food provision = 2000/80 = 200/8
⇒ 25 days
The food provision will last for 25 days, if 30 more girls join.