Given:
3 red dresses
2 white dresses
1 blue dresses
Probability that she will wear the blue dress at her party.
3 + 2 + 1 = 6 total number of outcomes.
There is only 1 blue dress so the number of ways it can happen is only 1.
probability of an event happening = number of ways it can happen / total number of outcomes
Probability = 1/6 or 0.166 or 16.6%