A class records the eight scores 5, 5, 6, 6, 7, 7, 8 and 8. What is the mean score?

Explication

The eight scores total 52, and dividing by the 8 scores gives a mean of 6.5. Answering 13 comes from dividing 52 by the 4 different score values that appear, but every repeated score counts separately, so the divisor is 8 rather than the number of distinct values.