Six readings were recorded as 3, 8, 10, 12, 18, 21. Later it was found that the reading 10 should have been 16 and the reading 18 should have been 12. What is the effect of these two corrections on the mean and on the median?

One correction adds 6 to a reading and the other takes 6 away, so the total stays 72 and the mean stays 72 ÷ 6 = 12. The ORDER does change: the corrected values in order are 3, 8, 12, 12, 16, 21, so the middle pair moves from 10 and 12 (median 11) to 12 and 12 (median 12). Equal and opposite corrections always leave the mean alone, but they can still shift the middle of the data.