## Poll

 1/4 1 vote (1.72%) 1/3 No votes (0%) 1/2 14 votes (24.13%) 2/3 39 votes (67.24%) 3/4 No votes (0%) Not information information given. 1 vote (1.72%) I believe in the "other math." No votes (0%) I'm a bigot 1 vote (1.72%) I'm a bully No votes (0%) I miss Vermenti 2 votes (3.44%)

58 members have voted

JoePloppy
Joined: May 2, 2014
• Posts: 82
May 15th, 2014 at 8:07:43 AM permalink
This thread was an eye opener for me. This question is a perfect example of how something counter intuitive is true explained by math. It was like those old 3D puzzles you stare at, then a crazy 3D pirate ship appears.
I just couldn't let the last word be incorrect.
2/3
RS
Joined: Feb 11, 2014
• Posts: 8623
May 15th, 2014 at 8:15:21 AM permalink
The question is worded poorly. I understand why one would say 2/3, but the question suggests the white side must be observed (and the black side cannot be observed).

If black side can be observed, then it's 2/3.
If only white side can be observed, then it's 1/2.
arcticfun
Joined: Oct 2, 2013
• Posts: 175
May 15th, 2014 at 8:16:18 AM permalink
Quote: sonuvabish

The coin is always white when you pull it out, 100% of the time. It is immaterial which white side comes out on the all-white coin because you are not observing 2 coins. That means there is a 50% the other side is white. Re-read the question. 2/3 is the probability that one of the remaining 3 faces is white; you either have the all-white coin, or you don't.

2/3 is simply the ratio of white faces to total unseen faces. This is the initial thought that everyone should have; everyone knows the chance of randomly selecting a white face, excluding the given, is 2/3--that was learned in elementary. If this is actually the correct answer, it is a trick question with poor wording. I would not call that controversial.

False. This is very similar to the Monty Hall problem (check wikipedia). Three closed doors - one has a car, the other two have nothing behind them. You point at one of the three doors. The host walks to the doors and opens one of the other two doors, and reveals that the car isn't behind that one. You can now choose to either keep your door or switch doors. What do you do? Open your door or open the other one? Straight from the movie 21! You can't ignore the information you're given.
JoePloppy
Joined: May 2, 2014
• Posts: 82
May 15th, 2014 at 8:22:41 AM permalink
Quote: RS

The question is worded poorly. I understand why one would say 2/3, but the question suggests the white side must be observed (and the black side cannot be observed).

Hahaha, what question are you reading?

Quote: Wizard

There are two coins in a bag. One is white on one side and black on the other. The other is white on both sides.

You randomly draw one coin, and observe one side only, which is white.

What is the probability the other side of that coin is white?

That sounds like you randomly draw one coin, and observe one side only, which happens to be white.

2/3
2/3