A snail is at the bottom of a 20-meter pit. Every day the snail climbs 5 meters up but at night it slides down 4 meters.
How many days does it take before the snail reaches the top?
REVEAL THE ANSWER
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. And, 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. And, 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 pit.