Braingle: 'The Truel' Brain Teaser

The Truel

Probability puzzles require you to weigh all the possibilities and pick the most likely outcome.

Puzzle ID:	#22672
Fun:	(2.84)
Difficulty:	(2.39)
Category:	Probability
Submitted By:	brianz

Mr. Black, Mr. Gray, and Mr. White are fighting in a truel. They each get a gun and take turns shooting at each other until only one person is left. Mr. Black, who hits his shot 1/3 of the time, gets to shoot first. Mr. Gray, who hits his shot 2/3 of the time, gets to shoot next, assuming he is still alive. Mr. White, who hits his shot all the time, shoots next, assuming he is also alive. The cycle repeats. If you are Mr. Black, where should you shoot first for the highest chance of survival?

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Answer

He should shoot at the ground.

If Mr. Black shoots the ground, it is Mr. Gray's turn. Mr. Gray would rather shoot at Mr. White than Mr. Black, because he is better. If Mr. Gray kills Mr. White, it is just Mr. Black and Mr. Gray left, giving Mr. Black a fair chance of winning. If Mr. Gray does not kill Mr. White, it is Mr. White's turn. He would rather shoot at Mr. Gray and will definitely kill him. Even though it is now Mr. Black against Mr. White, Mr. Black has a better chance of winning than before.
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Comments

Melly13 May 05, 2005	Ok that brain teaser is very confusing! TO many names... but it's not a bad one
phrebh May 05, 2005	The teasers are getting violent.
catzgirl_95 May 05, 2005	EZ but fun
markmonnin May 05, 2005	I didn't get it right... but I enjoyed the answer!
Ozymandias May 05, 2005	Good one!!
norcekri May 05, 2005	Considering that this is an old classic from the literature, I'm surprised to see this here. Don't the editors know about copyright violation?
vikingboy May 06, 2005	not only is it an old story, but I also think it's highly questionable. The point is, Mr Black is hoping that Mr White is taken out of the picture. By shooting the ground, he hopes that Mr Gray will shoot at Mr White, with a 67% chance of hitting him. If Gray does kill White, then black has a 33% chance of killing Gray, followed by Gray having a 67% chance of killing Black, this goes on until one dies. If Black shoots the ground, and Grey does not kill White, White then kills Grey, and Black has a 33% chance of killing White, if Black misses, White kills him 100% of the time. If I'm Mr Black, I take my shot at Mr White. 33% of the time I kill him, and now it's me against Mr Gray and I have a chance. 67% of the time it reverts back to the previous scenario of Grey vs White, and nothing changes. Give me that added chance.
brianz May 06, 2005	Here's the complete solution, vikingboy. If Mr. Black shoots the ground, there is a 1/3 chance that he will be left with Mr. White and a 2/3 chance he will be left with Mr. Gray. If he is left with Mr. White, he has a 1/9 chance of survival. If he is left with Mr. Gray, his probability of survival is 1/3((2/9)^0+(2/9)^1+(2/9)^2+(2/9)^3...) which is an infinite series that approaches to 3/7 (I think). Therefore, If Mr. Black shoots the ground, his total chance of survival is 1/9+3/72/3=1/9+2/7=25/63 Now, if Mr. Black shoots Mr. Gray, there is a 1/3 chance that he will be left with Mr. White and die and a 2/3 chance that he will miss and that we will go into the same situation as if Mr. Black just shot the ground. Therefore, his chance of survival is 25/632/3=50/189 If Mr. Black shoots Mr. White, there is a 2/3 chance that he will miss and he will have a 25/63 chance of survival. There is also a 1/3 chance that he will kill Mr. White and we will go into the situation with just Mr. Gray and Mr. Black, except this time Mr. Gray shoots first. Therefore, Mr. Black has a 1/3*2/7 chance of survival. So his total chance of survival is 2/21+50/189=68/189 When comparing the three fractions, the chances are 75/189, 50/189, and 68/189. Therefore, Mr. Black should shoot the ground.
brianz May 06, 2005	Okay, I know that was kinda complicated, but think about it this way. If Mr. Black is left with Mr. Gray and Mr. Black gets to shoot first, there is an unkown probability of survival, let us say x. If Mr. Black shoots the ground, his chance of survival is 1/9+2/3 x. If Mr. Black shoots Mr. Gray, his chance of survival is 2/3(1/9+2/3 x), so he should not shoot at Mr. Gray. If Mr. Black shoots Mr. White, his chance of survival is 2/3(1/9+2/3 x)+1/31/3x. Now we must prove that 1/3(1/9+2/3 x) is more than 1/31/3x. First we can simplify to try to prove 1/9+2/3 x > 1/31/3x. We can deduce that x is slightly larger than 1/3, and we know that 1/9+2/31/3 > 1/31/31/3, so we know this will always be true for any value of x greater than 1/3.
bobthesnail May 06, 2005	Slightly easier (I think) way to look at it. You are Mr. Black. Do you want to aim at Mr. Gray? Let's say you hit him. Then you instantly die from Mr. White. Survival rate from hitting Mr. Gray = 0 Do you want, then, to aim at Mr. White? Let's say you hit him. Then it's Mr. Gray vs Mr. Black with Mr. Gray getting the first shot. Since there's a 2/3 probability in this case of Gray killing Black with the first shot, Black's chance of surviving such a duel is less than 1/3. Let's say you miss. Then, Mr. Gray's turn to aim. If he aims at Mr. Black, and hits him, he instantly dies. Thus, aiming at Mr. Black is stupid. Let's say Mr. Gray aims at Mr. White. If he hits Mr. White, he then is in a 2 person duel with Mr. Black. Let's say he misses. Then Mr. White has his choice of killing Black or Gray. Either way, his opponent gets one chance to take him out. Thus, Mr. White would kill Gray. Gray, knowing that, will not shoot at the ground, knowing that White would instantly kill him. Thus, Gray will shoot at White. When it gets back around to Mr. Blacks turn, only one opponent will be left. Either way, Mr. Black gets AT LEAST a 1/3 chance of shooting that person, as opposed to the LESS THAN 1/3 chance that he would have if he were in a 2 person duel with Gray and Gray shooting first. (Mr. Black has exactly 1/3 chance of winning a duel between him and White if Grey had killed White. He has a slightly better chance if he's facing Gray, since Gray's aim isn't 100% and Black gets a second turn.) Interesting side note: logic stays the same if Gray's aim goes up to 100% and Blacks goes infinitely close to 0.
xsadclowneyezx May 06, 2005	foxxie
celebrity May 06, 2005	I guess I liked it it was okay...
vikingboy May 06, 2005	alright, don't get your thongs all twisted...I understand the odds just fine, what I overlooked was that if Black does kill White, Gray gets the first shot at Black. If he shoots the ground AND the others do as expected Black gets to shoot first. Keep in mind that you are assuming that the other two have the same information you do, and that they act logically. Still say that this one has been around forever.
Master_Yoda May 06, 2005	What if Mr. Gray or White decide to shoot and Mr. Black. Then Mr. White would have the better chance of surviving because Mr. Black would shoot at the ground and Mr. Gray would shoot him. leaving Mr. White to shoot Mr. Gray. Too many "if's"
brianz May 07, 2005	But since Mr. White is better than Mr. Black, Mr. Gray would rather shoot at him for his highest chance of survival, Master Yoda.
cracker May 07, 2005	It was confusing! What is a truel anyway? vikingboy, lay off being too smart. You are making me look pretty stupid, I can't figure out these confusing riddles. It isn't my fault that I am not as smart as ya'll!
changegurl May 07, 2005	I just use plain on common sense on this. I so! proud of MYSELF!!!!!!!!!!!!!!!!!!!!!!!
brianz May 07, 2005	Cracker, a truel is supposed to be like a duel except there are three people fighting in it. However, it is not actually a real word.
libra0890 May 07, 2005	SOOOOOOOOOOOOO STUUUUUPPPPIIIIIIIIDDDDDDDDDDDD. SO DUMB
Question_Mark May 07, 2005	I guessed the air. Ground, air, its all the same to me.
darthforman May 08, 2005	my head hurts
zonahobo May 09, 2005	Someone should shoot Mr. Black for getting in a fight with two guys who are such better shots than him .. classic or not, as long as it doesn't duplicate another post .. what's the harm .. I enjoyed it.
jeSuS123 May 09, 2005	i didnt kno ground was a option
Maimai May 12, 2005	I feel ultra stupid. I have a head ache... Way too many names... I think Mr. Black should just shoot everybody three times and not follow the rules. Y'know... just a suggestion...
Javian May 18, 2005	Good one...made you think
sniperkid990 May 24, 2005	my brain is pulsing toooooo much informatin to proses
Poker Jun 10, 2005	You think three people has a strange outcome? I tried to figure it out for four (Mr. Red with 1/4, Mr. Yellow with 2/4, Mr. Green with 3/4, and Mr. Blue with 4/4), and there's a very strange result. Mr. Yellow has the best odds, followed by Mr. Green, Mr. Red, and Mr. Blue, in that order, if everyone follows ideal strategy. Mr. Red fires at Mr. Blue. If he misses, Mr. Yellow fires - but not at Mr. Blue or even at Mr. Green, but at Mr. Red! If he misses, Mr. Green fires at Mr. Blue. If he misses, Mr. Blue bumps off Mr. Green. In any case, once it's down to three people, it's the same strategy as the truel shown here (even if Mr. Blue was bumped off, although it was significantly harder to figure out).
brianz Jun 11, 2005	That seems like a pretty interesting problem, Poker. I'll try to figure it out some day. Note, however, that in this problem, we aren't trying to figure out who has the highest probability of staying alive, but rather where Mr. Black should shoot to have the highest probability of staying alive. In my problem, Mr. Black has a 75/189 chance of staying alive, Mr. Gray has a 8/21 chance of staying alive, and Mr. White has a 2/9 chance of staying alive. Therefore, Mr. Black has the highest chance of survival. Additionally, in your problem with 4 people, I think it would be better for Mr. Red to shoot at the ground too, as this would guarantee a 1 on 1 match with Mr. Red against someone else, with Mr. Red having the first shot.
Poker Jun 15, 2005	Not quite, since then it would be down to three people, not two. Red actually has better odds if he kills Blue than if he doesn't kill anyone. I've got to warn you, though, it's tricky to figure out (especially for Red, Yellow, and Green left, since they all sometimes miss - at least, that's what I thought when I first came to that point). And some of those fractions can get pretty large. For example, at the start, Blue wins 21/256 of the time, Yellow wins 11727/28840 of the time, Green wins 40257/149968 of the time, and Red wins 2914283/11997440 of the time! If this is what it's like with four, with five the fractions must be huge!
Wendy510 Jun 23, 2005	it's good and complicated, but how come so many of you people are trying to explain it when it's already a good explanation in the answer?
dolphingurl12 Jul 15, 2005	What I wanna know, it how do they know how often they hit or miss cause this sounds like something you can only do once.
jhonertwert Aug 14, 2005	i wanna shoot the person who rote that
shoulder_angel Sep 22, 2005	Wow! How do you think of things like that? It all makes my head spin!
realm2346 Oct 31, 2005	At first, I said that Mr. Black should shoot Mr.White because he never misses his target......then I said hit the ground.
legodude Dec 02, 2005	I personally think that this teaser is awesome. It makes you think (sadly, I did not solve it correctly because I overlooked the possibility of shooting the ground). -An Interesting Note- I've discovered (with the necessary aid of a graphing calculator) that the limit of x^0+x^1+x^2+x^3+x^4+x^5+x^6... where 0≤x≤1 is (-1)/(x-1) . -Comments on the Above- The boundaries for x (0 and 1) are inclusive in this case, but inserting the boundaries into the equation wields 1 and 1/0=∞*, respectively. The solution for x=0 exists because 0^0+0^1+0^2... =0^0=0/0, which can equal any real number (I also discovered this, but I found out that it had already been found out) (say a=0 and b is real, therefore ab=0, b=(0/a)=(0/0), thus 0/0 can equal any real number), equals, in this case, 1. As for the second boundary, you would expect 1^0+1^1+1^2...=1+1+1... to equal ∞. If you would rather not go into the abstract realm of division by zero, just change both inequality signs to
legodude Dec 02, 2005	(cont. from above)
legodude Dec 02, 2005	less than signs. * I say this tentatively, because I think that it is generally accepted. I have not experimented enough with the concept of infinity to feel certain that it is equal to 1/0. It's an interesting topic, though.
paul726 Dec 11, 2005	I have to agree that Black should aim at White with his first shot. Unless he aims at and kills Gray with his first shot, he will get to shoot again. (unless either of the others is stupid enough to shoot at Black while their other opponent is still breathing) In any case, brainz, I disagree with your post of May 5 where you state that Black has a 1/9 chance of survival vs. White. His chance is 1/3. If he hits his only shot, he wins, if he misses, he's dead. This is assuming, of course, that he's facing White because White wisely took out Gray with his first opportunity.
SPUTNIK2 Jan 20, 2006	that was a real blast I gave it my best shot! thanks, good teaser
Jimbo Feb 25, 2006	An oldie but a goody. Teasers with completely unexpected answers are always more interesting. And besides, we'll all have to go and investigate the quadruel won't we? Watch this space!
shadow-x Mar 03, 2006	ok i think that he should shoot at the guy who always hits cuz he would probly miss, and then the other guy would shoot at the guy who always hits then he wuold be dead;....
buenos May 09, 2006	This teaser isn't racist...... right...?
elduce Nov 03, 2006	Isn't is a 'dual' and not a 'truel'? I looked up 'truel' and it doesn't exist.
elduce Nov 03, 2006	Isn't it a 'dual' and not a 'truel'? I looked up 'truel' and it doesn't exist.
extremetigerfan Jul 26, 2007	Isn't shooting at Mr. White and missing the same as shooting the ground? Why not take the 1/3 chance that you might actually kill the most potent person first? does your probability of dying go up if you kill White on the first shot? I don't want to work it all out, just tell me.
zembobo May 21, 2008	No, shooting Mr White and hitting him gives Mr Grey only one target...YOU. You want Mr white alive because you can safely assume Mr grey will shoot at him and probably hit him. If Mr Grey misses, Mr White will shoot him first giving himself a 1/3 chance to live. My biggest problem with this teaser is that shooting the ground was NOT an option. It clearly said that they were taking turns shooting each other, not the ground. Still, got me thinking.
zembobo May 21, 2008	I meant to say in my previous comment..."giving himself a 2/3 change to live", not "1/3" sorry about that..guess I should proof read a little better.
bbbz Jul 20, 2008	it actually doesn't matter who or what black shoots at because if you've seen the movie, "the white, the gray and the black" you would know that mr white takes the bullets out of mr blacks gun the night before.
donga Aug 19, 2008	good one!!! i enjoyed the question and the explanation is correct but something is boggling me, why should Mr.white target Mr.Grey ? definitely Grey has more chance of killing him than black , but we are dealing with probability so white will choose Grey 2/3 times and black 1/3 times . but in the case of Grey , choosing black will mean instant death , so he always targets white.
opqpop Jan 26, 2010	donga, suppose you are white. If you shoot at black and kill him, it becomes you vs. gray. If you shoot at gray and kill him, it becomes you vs black. Since gray is a better shooter, it is clear you would prefer the latter case: you vs black. Hence, white always shoots for gray.
Ricorum Nov 23, 2010	If Mr. Black only hits what he's aiming at 1/3 of the time, does that mean he has a 66% chance of missing the ground?
No_Eyed_Fsh Nov 23, 2010	Even the worst shooter should hit the ground... I think.
builder Nov 23, 2010	Shooting the ground is a guaranteed hit. Wouldn't that mean the next two shots would miss their target. Might as well try to take out white. Actually each shot has the same chance of hit or miss. There is no guarantee that in 3 shots one will be a hit.
angitude21 Nov 23, 2010	Someone has way too much time on their hands!!! This was beyond stupid and who cares!!!
TallTimber Nov 23, 2010	I lost interest way before getting to the end of the question.
patiencewithaP Nov 23, 2010	Good teaser! But it made my head hurt! I say Messrs. Black, White, and Gray should put down their guns and go have a drink together instead.
yash23 Nov 23, 2010	I find this racist XD
dodgerh8ter Nov 23, 2010	Hey, why am I Mr. Pink? Because you're a f*****. Why can't we pick our own colors? No way, no way. Tried it once, doesn't work. You got four guys all fighting over who's gonna be Mr. Black, but they don't know each other, so nobody wants to back down. No way. I pick. You're Mr. Pink. Be thankful you're not Mr. Yellow.
jcann Nov 23, 2010	This is one of the worst teasers I've ever seen on this site, as are also some of the comments. (Especially the one above.) I used to enjoy coming to this site every day, but it doesn't seem the same anymore.
crazyfunkychik Nov 23, 2010	I think he should have shot Mr.White first because Mr.White never misses so that would be a relief to get rid of him but i do see your point now
2big2fail Nov 24, 2010	if i were mr. black i'd shoot myself in the foot and call it a day. after all, the question is about survival, not about winning.
j_winburn Nov 24, 2010	Hey, if Mr. Black only hits his target 1/3 of the time, does that mean if he shoots at the ground he will probably miss? Where will the errant shot go? Could he accidentally hit one of the other two? Or himself?!!?