Trickier version of popular 'Snail and wall' interview puzzle

#Hard
#Tricky
#Puzzle
#riddle
You may already know the snail and wall puzzle... but in this video I am going to show you
a trickier version of this puzzle that can easily trap anyone in an interview.

The interviewer will first ask you the basic question:

There's a 10 meter high wall. A snail is at the bottom of the wall ... it climbs up 2 meters in one hour ...
Then it rests for the next hour... and while resting, it slides down one meter... it continues the same pattern until it reaches the top.

How long will it take for the snail to reach the top of the wall....

Of course the interviewer is expecting you to tell the wrong answer as 20 hours... because... Eventually, for every two hours it climbs up just one meter...
so to climb up 10 meters height, it would take 20 hours...

But since you know how to solve this puzzle... you would explain that
by the end of 16 hours.. it has climbed up 8 meters...
and in the next hour... it climbs up -the- remaining 2 meters to complete 10 meters... and it just reaches the top of the wall.
And now it doesn't have to slide back again... because it has already reached the top.

So.... the answer is .. 17 hours.

The interviewer would congratulate you for this correct answer... and he would continue with his next puzzle.

Now the snail is at the top of the wall....
If it follows the same pattern of climbing down for one hour and resting for next hour...and so on... how many hours does the snail take to reach the bottom of the wall?

I'd like to thank Tamir Erez for sharing this deadly interview trap.

I highly encourage you to pause the video and think logically.
If your answer is six and a half hours... then you have been successfully trapped by the interviewer...
The solution is trickier than expected.

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You are most welcome to share puzzle, math problems or any topics for upcoming videos.

[axiezimmah]

It's really bending physics because that's not how snails work, or gravity for that matter.

[Mackem-blix]

That’s not how gravity works. It wouldn’t add a speed component, but one of acceleration. So it would be constantly accelerating downwards and would take far less time to reach the bottom.

[schivver]

I thought it is 0 hours because the snail can just fall from the top

[prakharsharma]

Well it is obviously correct, but the method to solve these, is to use only the information which are given to us, cuz then in every question, many possibilities and forces can be possible

[MrScarabus]

6.5 hr is also correct answer. Let me explain.

At 2:26 you"observe" the snail and say that is slide down due to gravity. Ok. Then you MAKE AN ASSUMPTION that while snail moving it also slides down. In this case answer will be 4 hr.

BUT. The statement of the problem does not indicate whether the snail slides down when moving or not.

So, you may also LEGITIMATE ASSUME that while moving snail has constant speed 2 m/hr in any direction (due to 100% friction) and slides down only during rest (it just chillin'). Then answer will be 6.5 hr.

It's not a trick or trap, it's just a problem with incomplete conditions when answer depends on YOUR ASSUMPTION. Think wider.

[manshulgarg]

For all those thinking this explanation to be wrong, must rethink because it is correct solution and admire the beauty of this concept.

[draughtoflethe]

I kind of disagree. When one climbs downward, one is actually fighting against gravity in order to make a controlled descent; otherwise, snail slime or no, the answer would be "just over one second, with a distinct *splat* at the end." So I'm not convinced that the snail should get to add gravity to its active climbing speed.

[NoriMori]

I don't think this puzzle is "tricky". I think it moves beyond "tricky" into "unfair". The most important rule of logic puzzles is to not assume any information that you weren't given. You are only told that the snail can travel 2m in one hour, and slides down 1m in one hour. You are not told that his 2m travel rate is dependent on gravity, or that it can change depending on whether he's moving up or down. Therefore there is no reason to assume such a thing.

Second issue is that the second version changes the genre of the puzzle without telling you (from an abstract logic puzzle to what is arguably a _situation_ puzzle). And the third issue: Even if one is told that they have to think about the problem in terms of physics, if you don't specify the parameters, they could come up with any answer and justify it with physics. After all, it's been a long time since the snail ate, so maybe he's going to move slower as time goes on. Maybe it started raining. Maybe we should think about moisture and air pressure. Or maybe the snail was smarter than most and decided to simply detach itself and fall the 10m in a few seconds. Without specifying the parameters in advance, these considerations are no less valid (or rather, no more invalid) than the consideration of gravity.

[lionelmessi]

I was trapped in second puzzle which was really awesome

[CultOfJ]

Correct answer finally 😁 without knowing the answer honestly 😎🤟👍

[gautamsinghal]

The second explanation was a bit bizzare because gravity is a force and force will accelerate you downwards rather than providing with a constant velocity...

[dey]

Well...snail sleeps bcoz it gets tired going upwards. While going downwards, the snail won't sleep as it won't get tired. So it will reach the ground in 10/4=2.5 hrs.

[paulmeyer]

I thought 10 hours! Going downwards it's harder to keep traction than going upwards, so until further notice 1m/h is the fastest downward speed.

[michaelcj]

Great twist to the puzzle, but the whole story/explanation raises a lot of eyebrows, as you can see on the comments.

It would be better to use a different case, for example a boat moving full-power upstream with a velocity of 20km/h but has to shut down its overheated machine every hour for 1 h, where it slides back 10km. How long does it take for the boat to reach its destination which is 100km away?

And then the trick question: How long does it take for the same boat to go back home on the same path, travelling at the same full power?

This would make a lot more sense.

Snail speeding up due to gravity... kinda far-fetched for me -_-"

[nidadursunoglu]

Gravity isnt the same at differnt heights
snail is going to lose some mass cuz it produces that sticky thing i dunno so gravity effect will change again
also if its going the exact same path down it will face those sticky things it had produced so itll slow down.
Maybe it changed its mind and wants to go slower /faster this time.

[ashisharma]

Don’t agree, its not a free falling object, how can simple gravity rules apply? How about the force and friction?

[Susp]

Subtitles: What's up illogical people? This isn't Amar



Edit:It actually shows: This is kamar

[christianfunintuscany]

In my opinion the solution of second part of puzzle n.1 is not compatible or consistent with the one of the first part. In order to create a trap for the interviewer I think the questioner falls into a nonsense. In the first part the snail climbs up for 2m in one hour, then slides down for 1m in one hour and so on till he reaches the top (10 m). Solution is 17 hours, so this means that while climbing up the snail is not decelerated by gravity, otherwise we should reduce its speed of 1 m per hour, that’s what happen when it sleeps. And the result would be that the snail never mooves (1 m up, 1 m down, 1 m up ...). So if the snail moves up and its speed is 2 m per hour, and we all agree that he gets the top in 17 hours, it is not possible that in the second part its speed is affected by gravity on the way back. Or gravity acts in the two cases or never. So we have to simply consider the snail is climbing down at 2 m per hour, with no effect of gravity, and 6, 5 hours turns to be the most logical answer.

[abdulirfan]

The snail can even slip and fall down .... LOL 😂

[dougaltolan]

Snail climbing down speed is not specified, your assumption is simplistic.
It's entirely possible that the snail has a maximum speed, limited by how much slime it can make.