A snail is at the bottom of a 20 meters deep pit. Every day the snail climbs 5 meters upwards, but at night, it slides 4 meters back downwards.
How many days does it take before the snail reaches the top of the pit?

The snail reaches the top of the pit on the 16th day!
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.
...