After 30 days, there will be enough food left to sustain 300 people for 90 days – 30 days = 60 days.
After the 100 people leave, there will be 300-100 = <<300-100=200>>200 people left.
The 200 people will eat 200/300 = 2/3 as much food as the original group of people in the castle.
The 60 days' worth of food will last this smaller group for 60 days / (2/3) = <<60/(2/3)=90>>90 more days.
#### 90