It is a minimal classic against haste. Almost everyone does the math too quickly and gives the snail a night it no longer needs.
The snail in the well
Riddle statement
A snail climbs 3 meters each day and, while sleeping at night, slides 2 meters.
If the well is 10 meters deep, in how many days will it come out?
Show solution
Solution
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 it no longer slides, because as soon as it reaches the top it 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 gain 1 net meter. Thus, after the night of the 7th it is 7 meters.
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's why the correct answer is 8 days.