The first person can be any one of 6.
For each of those . . .
The second person can be any one of the remaining 5.
For each of those . . .
The third person can be any one of the remaining 4.
Total number of ways to pick 3 people out of 6 = (6 x 5 x 4) = 120 ways.
BUT . . .
For each 3-person team, there are (3 x 2 x 1) = 6 ways to pick
the same three people.
In the 120 ways to pick 3 people, you'll find each different
group of 3 people picked 6 times, in a different order.
The number of different 3-person teams is
120 / 6 = 20 teams .
