A dairy sold an average of 180 litres of milk a day over the first 10 days of a 30-day month. Over the whole month the average was 200 litres a day. What was the average daily sale over the remaining 20 days?

The month sold $30\times 200=6,000$ litres and the first 10 days sold $10\times 180=1,800$, so the last 20 days sold 4,200 litres, an average of 4200/20 = 210 a day. Quicker: the first 10 days ran 20 litres a day short, a shortfall of 200 litres, and making that up over 20 days needs only 10 extra a day, so 200 + 10 = 210. 220 makes up the shortfall at 20 litres a day, as though the two stretches were the same length; the second stretch is twice as long, so it repairs the gap at half the rate. 190 moves the correction the wrong way, though the early days were the weak ones. 4,200 is the total for the 20 days, not the daily average.