A large gathering occurred at the town hall with 200 people participating. 100 people decided to have a snack, and then 20 new outsiders joined in to have a snack. Half of these snack eaters got full and left. 10 new outsiders came to have snacks, too. 30 more snack eaters got their fill and left. Then half of the remaining snack eaters left later on. How many snack eaters are left? Let's think first. Chain of thought:
At the meeting, 200 - 100 = 100 people decided to have a snack. 20 new people came in to snack and the area had 100 + 20 = 120 of them eating. 120 / 2 = 60 people got full and left. 10 more new people came to join in the snacking so 60 + 10 = 70 people were eating. After 30 people left, 70 - 30 = 40 people who were eating remained. In the end, only 40 / 2 = 20 kept eating.
Therefore, the answer is 20.