A snail is at the bottom of a 20 meters deep well. 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 well**?**

**Answer:**

The answer is actually 16 days. Why?

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 well, this time, not sliding back.