Left-skewed distribution

A left-skewed graph piles up on the right and trails off to the left, because the tail is what names the skew. Those few small values in the tail pull the mean down while the median stays put, so the mean typically comes in below the median. For the values 2, 18, 19, 20, 21 the mean is $80/5 = 16$ while the median is 19. Scores on an easy test often look this way, with most students bunched near the top and a handful far below, but treat the mean-below-median pattern as a tendency rather than a rule.