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A 2.5m deep water tank has water poured into it for 8 hours, starting at 9am. The water level rises by 60cm in this time. However, it seems that water is leaking out again at night, because the next morning the water level has dropped by 20cm.
If the water level continues to rise 40cm a day in this way, on what day will the tank first overflow?
That's right! The answer is the 6th day.
It's easy to fall into the trap of thinking that because the water level rises by 40cm each day, it will ahve risen to 2.40m by the 6th day, and therefore overflow the 2.5m deep tank on the 7th day. However, on the 6th day the water level actually starts at 200cm and rises by 60cm over the course of the day, overflowing the tank.
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