Chapter 6.2, Problem 33E

Amusement On a particularly boring transatlantic flight, one of the authors amused himself by counting the heads of the people in the seats in front of him. He noticed that all 37 of them either had black hair or had a whole row to themselves (or both). Of this total, 33 had black hair, and 6 were fortunate enough to have a whole row of seats to themselves. How many of the black-haired people had whole rows to themselves?