What is the average of all the multiples of 6 that lie between 50 and 100?

The multiples of 6 between 50 and 100 are 54, 60, 66, 72, 78, 84, 90 and 96 - eight numbers, evenly spaced, so the average is the midpoint of the ends: (54 + 96)/2 = 75. (Their total is 600 and 600/8 = 75.) 72 slips in 48 as well, giving nine numbers from 48 to 96 and an average of 72; but 48 is below 50. 78 starts the list at 60, forgetting that 54 is already above 50. 100 divides the correct total of 600 by 6, the size of the step, rather than by 8, the count of numbers.