Dogs in the GoodDog Obedience School win a blue ribbon for learning how to sit, a green ribbon for learning how to roll over, and a white ribbon for learning how to stay. There are dogs in the school.
62 have blue ribbons, 55 have green ribbons, and 63 have white ribbons.
31 have a blue ribbon and a green ribbon; 32 have a green ribbon and a white ribbon; 38 have a blue ribbon and a white ribbon.
16 have all three ribbons
The answer is 6.