There are 26 - 10 = 16 girls. The number of ways to choose 2 girls from a set of 16 is C(16, 2) = 120. (Your class might be using a different notation for combinations, like
.)
The number of ways to choose 2 people (boys or girls) from a set of 26 is C(26, 2) = 325.
The probability of choosing 2 girls is

Example: how to compute C(16,2):
