Solution: ∵ three groups 180 people, the sum of the first and second groups is 20 more than the third group.

The third group has (180-20)/2 = 80 people.

So the number of people in the first and second groups = 180-80= 100.

Also, there are two people less in the first group than in the second group.

∴ 49 people in the first group; The second group is 5 1 person.