The average temperature of a hill town from Monday to Thursday was 21°C, and from Tuesday to Friday it was 23°C. If Friday's temperature was 26°C, what was Monday's temperature?

Monday to Thursday totals $4\times 21=84$ and Tuesday to Friday totals $4\times 23=92$. The two windows share Tuesday, Wednesday and Thursday, so the 8-degree gap is exactly Friday minus Monday: Monday = 26 - 8 = 18°C. (Tue + Wed + Thu = 92 - 26 = 66, and 84 - 66 = 18.) 24°C works with the raw averages, 26 + 21 - 23, forgetting that each window covers four days. 34°C adds the 8-degree gap to Friday instead of subtracting it, but the later window has the higher average, so Friday must be the warmer of the two ends. 22°C is just the midpoint of 21 and 23.