The snail in the well

1. Don't count every day as ending with a slip.
2. The key is to look at what happens on the day the snail reaches the edge.
3. After 7 nights it is at 7 meters; by raising 3 more on day 8, he exits.

Answer: It comes out in 8 days. Explanation: The typical mistake is to think only of the net advance of \(1\) meter per day and conclude too quickly that it will take 10 days. But on the last day he no longer slides, because as soon as he reaches the top he leaves the well. Let's see the process: - at the end of day 1 it reaches 3 m and at night it drops to 1 m;

• at the end of day 2 it reaches 4 m and drops to 2 m;
• at the end of day 3 it reaches 5 m and drops to 3 m. In general, after each night you earn 1 net meter. Thus, after the night of the 7th it is 7 meters away. On day 8 it rises 3 more meters and reaches 10 meters. At that point it has already come out, so it doesn't slip again. That is why the correct answer is 8 days.