# Brain teasers **URL:** \ **Category:** random\ **Created:** [September 19, 2004, 11:30pm UTC](https://forum.kirupa.com/t/brain-teasers/64782 "2004-09-19T23:30:48Z")\ **Posts on this page:** 20\ **Page:** 6
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 22, 2004, 10:11pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/101 "2004-09-22T22:11:51Z")
… let’s go back to the crow-ones 😛 :ne: ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 12:22pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/102 "2004-09-23T12:22:46Z")
They saw the whites of his eyes, who cares. ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 1:31pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/103 "2004-09-23T13:31:50Z")
> [@hificopymaster](#): > > Ooh okay, I’m an idiot. > > First C picks a door. The chance of the prize being there is 1/3. When A reveals one of the goats, the chance of the prize being in the other door increases to 2/3. So C should logically switch. Doh! I didn’t get that :-/ > First C picks a door. The chance of the prize being there is 1/3. > I’m cool with that. > When A reveals one of the goats, the chance of the prize being in the other door increases to 2/3. > :h: I don’t see why. There are 2 doors: the one chosen by C and the one that has not been chosen. There’s a posche and a goat left to find. Behind each of those doors, there’s a 50 % chance you will find either one of them, no? :hangover: ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 1:34pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/104 "2004-09-23T13:34:28Z")
yeah, the chance that C gets the porsche rises from 33,33% to 50%, thought that too ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 1:37pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/105 "2004-09-23T13:37:34Z")
You agree that the chance of the prize being behind his original door is 1/3 right? So the chance of the prize being in either of the other 2 doors is 2/3, since we know the prize must be in one of the 3. The other 2 doors form a subset of the original set, and the probabilities associated with that subset don’t chance. So when the host shows the contestant which of those two doors has a goat behind it, the only possible place for the prize (within that set) is behind the other door. Since the probability of the prize being in that subset is 2/3 and the probability of the prize being behind the door the host opened is 0, the entire 2/3 must go to that remaining unopened door. ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 1:40pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/106 "2004-09-23T13:40:51Z")
It’s like, if you have a deck of cards, and someone says to you, you win $100000 if you pick the ace of spades. So you pick one card at random. There’s a 1/52 chance you picked the right card. Pretty slim odds there. You also know there’s a 51/52 chance the right card is in the rest of the deck. Then the other guy says, “I’m gonna turn over 50 of these cards which are NOT the ace of spades, and leave one unturned. You can then choose to switch if you want.” The chance of the ace of the spades being in that other part of the deck remains 51/52, no matter how many cards the guy turns over. Try it, most of the time, the unturned card will be the card you want. [size=1][EDIT] Another way to think of it: In this deck of cards example, if you say that there’s a 50/50 chance you picked the right card, you’re saying that your original card will be the ace of spades 50% of the time, which is really unlikely considering you’re choosing out of an entire deck.[/size] ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 2:02pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/107 "2004-09-23T14:02:29Z")
I’m not convinced by your explanation. Here’s why: you’re comparing 2 probabilities that are completely different [list][_]The probability that you find the Porsche in the first place, as you said 1/3 [_]The probability that you find the Porsche knowing that it’s not behind one of the 3 doors.[/list]For me, there’s abslutely no reason to switch, because once one goat is revealed, there are 2 doors, 2 possibilities for each door, hence a 50 % percent chance you’ll find the Porsche. ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 2:11pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/108 "2004-09-23T14:11:42Z")
> [@hificopymaster](#): > > It’s like, if you have a deck of cards, and someone says to you, you win $100000 if you pick the ace of spades. So you pick one card at random. There’s a 1/52 chance you picked the right card. Pretty slim odds there. You also know there’s a 51/52 chance the right card is in the rest of the deck. Then the other guy says, “I’m gonna turn over 50 of these cards which are NOT the ace of spades, and leave one unturned. You can then choose to switch if you want.” The chance of the ace of the spades being in that other part of the deck remains 51/52, no matter how many cards the guy turns over. Try it, most of the time, the unturned card will be the card you want. > > [size=1][EDIT] Another way to think of it: In this deck of cards example, if you say that there’s a 50/50 chance you picked the right card, you’re saying that your original card will be the ace of spades 50% of the time, which is really unlikely considering you’re choosing out of an entire deck.[/size] That’s a good example. Funny 🙂 ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 2:39pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/109 "2004-09-23T14:39:57Z")
actually, i think i’m with ilyas here… i thought i understood it at first, but when you think about it, once a dud door is eliminated, there’s a 50/50 chance it’s in either. Essentially, you’re just picking between the two doors to begin with, since one of the duds is being removed. So, we have 2 doors left, one of them has a car, one has a goat. it doesn’t matter which door he picked to begin with, there are now 2 possible options, equally likely. no? ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 2:44pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/110 "2004-09-23T14:44:22Z")
Okay, look at all the possibilities: 1. door A = prize, door B = goat, door C = goat 2. door A = goat, door B = prize, door C = goat 3. door A = goat, door B = goat, door C = prize Let’s say you pick door A. You have a 1/3 chance of winning by sticking to your original choice. The host opens one of the other doors, with a goat behind it. Then the last door has a 2/3 chance of having the prize. The other 1/3 is when you have the prize behind your door and both of the other doors are goats. Do you see it now? ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 4:23pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/111 "2004-09-23T16:23:05Z")
You could also look at your chance of being wrong. When you choose a door, theres 2/3 chance that you’re _not_ getting the Porsche. I think that it’s pretty obvious that that chance doesn’t change when one of the other doors is opened. There’s still 2/3 chance that you’re wrong. So you should change doors, since there’s now only 1/3 chance that you’re wrong when you pick the other remaining door. ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 5:21pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/112 "2004-09-23T17:21:23Z")
I’d just be happy with the goat. Geez :puzzle: ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 8:55pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/113 "2004-09-23T20:55:19Z")
I dig out tiny caves, and store gold and silver in them. I also build bridges of silver and make crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs my help, yet many people are afraid to let me help them. Who am I? ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 9:02pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/114 "2004-09-23T21:02:26Z")
You’re _azzy_… ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 9:35pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/115 "2004-09-23T21:35:00Z")
> [@Krilnon](#): > > You’re _azzy_… Either that… …or a magic leprechaun :beam: ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 9:47pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/116 "2004-09-23T21:47:55Z")
keep guessing. i am not giving up the answer that easily ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 9:52pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/117 "2004-09-23T21:52:29Z")
a dentist ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 9:57pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/118 "2004-09-23T21:57:45Z")
> [@McGiver](#): > > a dentist I think we have a winner. ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 10:01pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/119 "2004-09-23T22:01:14Z")
yes we have a winner…sob you guyz are smart /sob ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex011/uploads/kirupa/original/3X/6/2/621e5c11736f46532526e61be85940af4230f3e5.png) [@system](https://forum.kirupa.com/u/system)\ **Post date:** [September 23, 2004, 10:02pm UTC](https://forum.kirupa.com/t/brain-teasers/64782/120 "2004-09-23T22:02:45Z")
The rich need it, the poor have it, if you eat it you die. What is it? [Previous page](https://forum.kirupa.com/t/brain-teasers/64782.md?page=5) [Next page](https://forum.kirupa.com/t/brain-teasers/64782.md?page=7)