How To Solve The Seemingly Impossible Escape Logic Puzzle

An evil logician takes Alice and Bob captive. They have a chance to escape if they can deduce the total number of trees in the prison. Can they figure it out?

Source of puzzle

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[CalliopePony]

Question: "Are there 18 or 20 trees?"

Answer: "Yes, there are 18 or 20 trees."

[ficklepickless]

Let's face it, real Alice and real Bob would be screwed

[flashbash]

“Can they escape with certainty”
Here’s my logic:
If the answer is no, there would be no solution, therefore the answer must be yes.
So, yes, yes they can.
That was an easy puzzle

[Corrupted]

Real life version would be 27 trees and the guy would just kill them anyway, but he just wanted them to feel like they could escape lol

[theunknownblock]

when the logician comes into her cell and asks her if there are 18 or 20 trees, she could just say "yes" and they would both be set free.

[xisumavoid]

Since they don't communicate with one another, how do they know who was asked first? Also if Bob or Alice are anything like me they never would of thought logically like this :-P

[PaulH]

I understand how this type of logic problem works, but I'm not convinced that the reasoning works in this case. Since Alice knows that she sees 12 trees, and since she knows that the total number of trees that she and Bob see is 18 or 20, then she knows from the very start that Bob sees either 6 or 8 trees. She doesn't need to go through the process of eliminating the possibility that Bob sees 0 trees, or 1 tree, etc.

Likewise, Bob knows that he sees 8 trees and that the total number of trees they see is 18 or 20, and so he knows from the very start that Alice sees either 10 or 12 trees. He doesn't need to go through the process of excluding 20, 19, etc.

[destroyercs]

Let's look what would it look like in reality.
Day 1
Alice:I pass
Bob:I pass
Day
Alice: I pass
Bob passed

[CamoB-ubmy]

I have an answer. Are there 18 or 20 trees?
Me: 18
L: NOPE
Me: but if there is 20 trees, there must be at least 18

[ragepoweredgamer]

"In reality, they were both average humans, and died of dehydration long before this type of critical, logical thought process crossed their minds. They were too busy complaining about not having an iPhone charger.

[amyethington]

Alice on day 5: Oh, so there are 20 trees!
Bob every day: Idk so imma pass.
Logician: *Too bad Alice, the correct answer was 18, you will both be trapped forever.*

[shreerangvaidya]

"Did you figure it out?"
Sarcasm.

[maverickjohnson]

A more important question is: when they get out, does Bob say "I knew you'd figure it out today" or "how the heck did you figure it out?"

[billpuppies]

A more functional version of the riddle is "less than 19 or more than 19?" it takes away the distraction that Alice starts out with more substancial knowledge that "Bob has 6 or 8" and Bob knows "Alice has 10 or 12".

[johns]

Day 1

Alice realizes Bob must see at least 2 trees.

Seriously? Why did she wait for Bob to "pass" to come to this conclusion? If she sees only 12 trees and the question was whether there are 18 or 20 trees then she would have realized Bob must see either 6 or 8 trees without waiting for Bob to "pass". Bob "passing" would not have provided any additional information to Alice.

[kenzabouayad]

Wouldn't Alice already know on day 1 that Bob has at least 6 trees? And wouldn't Bob know that Alice has at least 10?

[SeaWaterever]

We can also figure out there is no chance there will be two human smart enough to solve this riddle in a such stressing situation.

[RGC_animation]

Even if Alice and Bob weren't perfect logicians, they would still have a 50% chance of escaping, great prison logician.

[maximedition]

Wait, but if there are either 18 or 20 trees, wouldn't Bob immediately realise that Alice sees at most 12 trees

[ann-marieburke]

This doesn't work unless bob knows that Alice goes first and Alice knows she is going first