A Snail in the well 20m deep puzzle

A Snail in the well 20m deep puzzle

A snail is in the well, 20 meters deep.

During the day it crawls up 5 meters,
but at night with some dew it descends
another 4 meters while it sleeps.

How many days will it take to get up there?

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Answer :
The Snail will take 16 days to get out of the well.