A tank of capacity 6,000 litres is filled by a single inlet. The inlet delivers 120 litres a minute for the first 20 minutes, after which falling pressure cuts its delivery to 80 litres a minute for the rest of the time. How long does the filling take?

The first 20 minutes deliver $20\times 120=2,400$ litres, leaving 3,600 litres to come at 80 litres a minute, which takes 45 minutes. Total 20 + 45 = 65 minutes. 50 minutes ignores the drop in pressure (6,000 ÷ 120). 75 minutes applies the reduced 80 litres a minute to the whole tank, forgetting the strong opening spell. 70 minutes adds the full-rate figure of 50 minutes onto the opening 20, counting those 20 minutes twice over.