16 days, I cheated!
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. The second day, he reaches 6 m., but slides back to 2 m. The third day, he reaches 7 m., and slides back to 3 m. … The fifteenth day, he reaches 19 m., and ends at 15 m. The sixteenth day, he reaches 20 m., so now he is at the top of the pit!
Conclusion: The snail reaches the top of the pit on the 16th day!
What is bought by the yard and worn by the foot?