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?
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.