The average monthly rainfall at a weather station over June, July and August was 220 mm, and over July, August and September it was 195 mm. If June recorded 215 mm, how much rain fell in September?

June to August totals $3\times 220=660$ mm and July to September totals $3\times 195=585$ mm. July and August appear in both, so the 75 mm gap is exactly June minus September: September = 215 - 75 = 140 mm. (July + August = 660 - 215 = 445, and 585 - 445 = 140.) 190 mm subtracts only one 25 mm, as if the windows covered a single month rather than three. 290 mm adds the 75 mm, though the later window has the smaller average, so September must be the drier end. 195 mm just repeats the second window's average.