Solution to: Climbing Snail
On the first day, the snail reaches a height of 5 meters and slides down 4 meters at night, and thus ends at a height of 1 meter.
On the second day, he reaches 6 meters, but slides back to 2 meters.
On the third day, he reaches 7 meters, but slides back to 3 meters.
On the fifteenth day, he reaches 19 meters, but slides back to 15 meters.
On the sixteenth day, he reaches 20 meters, so now he is at the top of the pit!
Conclusion: The snail reaches the top of the pit on the 16th day!