Dr. Johnson bought a new plant for his aquarium that was growing very fast. Every day it was doubling in size. On the 30th day the plant covered the whole aquarium. On which day did it cover half of the aquarium?
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Answer: 29 days.
Since it doubles in size every day, then the fact that it covered the whole aquarium on the 30th day implies that it covered half of the aquarium on 29th day.