Chapter 10 Counting Methods and Probability - 10.2 Use Combinations and the Binomial Theorem - 10.2 Exercises - Skill Practice - Page 694: 11

The student accidentally calculated a permutation.

We know the equation for combinations: $$nC_r = \frac{n!}{r!\left(n-r\right)!}$$ Thus, we find: $$\frac{6!}{2!\left(6-2\right)!}=15$$ Thus, the student did not remember the $2!$ in the expression, treating it like a permutation.