Twelve readings, already in ascending order, are 5, 8, 11, 13, 14, 18, 21, 24, 27, 30, 34, 41. Taking the median of the lower six as Q1 and the median of the upper six as Q3, which list gives the five-number summary (minimum, Q1, median, Q3, maximum)?

The extremes are 5 and 41. With twelve readings the median is the mean of the 6th and 7th, (18 + 21)/2 = 19.5. The lower half is 5, 8, 11, 13, 14, 18, and being six values its median is again a mean of a middle pair, (11 + 13)/2 = 12; the upper half 21, 24, 27, 30, 34, 41 gives (27 + 30)/2 = 28.5. So the summary is 5, 12, 19.5, 28.5, 41. Picking single values at the 3rd and 10th places instead of averaging pairs gives 11 and 30; taking the 6th reading 18 as the median forgets that an even count has no single middle; and 14 and 27 come from counting the quartile positions from the wrong end of each half.