There are 180 people in the three groups, and the sum of the people in the first and second groups is 20 more than those in the three groups.

If three groups plus 20 people equals the number of people in one or two groups,

The sum of the number of people in the first group and the second group =( 180+20)÷2= 100 (person)

Number of people in the first group =( 100+2)÷2-2=49 (people).