As part of a student project for the 1994 Science Fair in Orange, Massachusetts, 28 horses were made to listen to Mozart and heavy-metal music. The results were as follows: 11 of the 28 horses exhibited some head movements when Mozart was played; 8 exhibited some head movements when the heavy metal was played; and 5 moved their heads when both were played. If a horse is chosen at random, what is the probability the horse exhibited head movements to Mozart or to heavy metal or to both?