At the meeting, 200-100=<<200-100=100>>100 people decided to have a snack.
20 new people came in to snack and the area had 100+20=<<100+20=120>>120 of them eating.
120/2=<<120/2=60>>60 people got full and left.
10 more new people came to join in the snacking so 60+10=<<10+60=70>>70 people were eating.
After 30 people left, 70-30=<<70-30=40>>40 people who were eating remained.
In the end, only 40/2=<<40/2=20>>20 kept eating.
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