A university is carrying out a survey to study the number of hours students sleep at night during exam week. A sample of 20 students was drawn, and these responses were obtained: 6.5, 5, 6, 5.5, 7, 4, 4.5, 5, 6, 5.5, 4, 7.5, 6, 7, 8, 5, 6, 6.5, 6.5, 5. What is the sample mean of the data?