The cards show how many kilometres each of 8 friends cycled this week. What is the median distance?

In order: 12, 15, 17, 19, 23, 26, 28, 31. With 8 values the median is the mean of the 4th and 5th: $\frac{19+23}{2}=21$ km. Using the 4th and 5th cards without sorting (28 and 12) gives 20; 42 forgets to halve.