Snail climbs out of the well

Question:

A snail is at the bottom of a well that is 20 feet in depth. Every day the snail climbs 5 feet upward, but at night it slides 4 feet back down. After how many days can the snail climbs out of the well?

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.

C

A

P

T

A

I

N

I

N

T

E

R

V

I

E

W

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Solution: Watch out! This is a classic trap. The obvious answer is to reason that since the snail nets 1 foot per day in its climb, it will work its way out of the 20-foot well after 20 days. Duh! The only thing that you need to do here is take your time and work the puzzle out day by day. In fact, the snail is not moving at 1 foot per day but moving up 5 then falling 4. This means that on day 1, it is up to 5 and then down to 1. Day 2 it is up to 6 and then down to 2. Day 3 it is up to 7 and down to 3. On the sixteenth day the happy snail will go up to 20 and not go down any more (because he is out of there!).

Answer: 16 days.

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