Mari has 3 friends who are available to go with her to the movies, but she only has 2 extra tickets. How many different choices does Mari have for which 2 friends to invite?

3 different choices

Step-by-step explanation:

This problem is a combination problem: She has 3 friends, and we want to know how many groups of 2 friends there are. We can solve this using the combination of 3 choose 2:

C (3,2) = 3! (1! * 2!) = 3*2/2 = 3 possibilities

If we call her three friends A, B and C, the groups that can be formed are:

A and B

A and C

B and C