A rich earl has become the owner of a piece of land, which to his dissatisfaction turned out to be nothing more than a big swamp. The earl wants to get rid of the swamp. A salesman advises him to use his fast-growing plants which can cover the swamp very quickly. "This plant doubles every day, tomorrow you will have two, the day after tomorrow four, etc. In 80 days, your swamp will be completely covered." The earl reacts: "80 days? This takes far too much time. Then just give me eight of these plants."

Question 1: What did the earl think?
Question 2: And what do you think?

The earl probably thought: "Then I only have to wait for 10 days." This is wrong. Because if you start with 1 plant, after 3 days you'll have 8. So buying 8 plants saves you only 3 days. Hence with 8 plants, the earl has to wait 77 days.