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 12-02-2010, 11:44 PM #1 captain dean stranger   Join Date: Dec 2010 Posts: 5 I need help on a probability problem For the life of me I can't figure out how to solve this and I am in "dire" need of the answer. "An object randomly changes between five different colors once a day. A person randomly guesses the objects color twice a day (not the same guess in one day). What are the chances that the person WON"T guess the objects color after 22 days?" I came up with .0061% Please give me a hand.
12-02-2010, 11:58 PM   #2
SwatSkills
newbie

Join Date: Jul 2010
Posts: 21
Re: I need help on a probability problem

Quote:
 Originally Posted by captain dean For the life of me I can't figure out how to solve this and I am in "dire" need of the answer. "An object randomly changes between five different colors once a day. A person randomly guesses the objects color twice a day (not the same guess in one day). What are the chances that the person WON"T guess the objects color after 22 days?" I came up with .0061% Please give me a hand.
I dont see what you mean!

 12-02-2010, 11:59 PM #3 captain dean stranger   Join Date: Dec 2010 Posts: 5 Re: I need help on a probability problem "not the same guess in one day" as in the person cannot guess "green" twice in one day, but has to guess two different colors. Last edited by captain dean; 12-03-2010 at 12:04 AM.
12-03-2010, 12:22 AM   #4
EvilSteve

Join Date: Mar 2006
Location: 2p2 afterlife
Posts: 13,198
Re: I need help on a probability problem

Quote:
 Originally Posted by captain dean For the life of me I can't figure out how to solve this and I am in "dire" need of the answer. "An object randomly changes between five different colors once a day. A person randomly guesses the objects color twice a day (not the same guess in one day). What are the chances that the person WON"T guess the objects color after 22 days?" I came up with .0061% Please give me a hand.
I assume at least he knows what the five possible colors are, so he's not guessing aqua and chartreuse while the object switches between green red blue orange and purple. Absurd problem otherwise. So, the object changes color each day and stays the same color throughout the day, it might be blue on day 1 then red on day 2, etc? Seems like the hard part here is understanding what the problem is. Could be stated more clearly but I think I understand.

Ok so he makes two (different) guesses per day, meaning on any given day 2/5 chance of guessing the correct color for the day. But you want the probability he guesses incorrectly for 22 days straight. On any given day he has a 3/5 chance of not guessing correctly, so 22 straight days of never guessing right is (3/5)^22 = .00001316.

 12-03-2010, 12:39 AM #5 captain dean stranger   Join Date: Dec 2010 Posts: 5 Re: I need help on a probability problem so that would be .0013%? it makes sense, but just to make sure, id love if other would confirm.
12-03-2010, 12:46 AM   #6
EvilSteve

Join Date: Mar 2006
Location: 2p2 afterlife
Posts: 13,198
Re: I need help on a probability problem

Quote:
 Originally Posted by captain dean so that would be .0013%?
Multiply by 100 to convert a probability to a percentage, so yes.

 12-03-2010, 12:48 AM #7 SwatSkills newbie   Join Date: Jul 2010 Posts: 21 Re: I need help on a probability problem I confirm!(but why 22 days please?)
 12-03-2010, 01:18 AM #8 captain dean stranger   Join Date: Dec 2010 Posts: 5 Re: I need help on a probability problem my friend and i have been guessing another friends shirt for 22 days. he only wears 5 different shirts, and we have never gotten it right. we are a statistical anomaly. I thought the word problem i gave seemed simpler to explain though, any way, thats interesting. and for the record, the answer i gave in my question on my first post was for 19 days not 22, so i did do the math right. sorry this may not make too much sense as im super tired but hey.... anyway, thanks much
12-03-2010, 09:29 AM   #9
Actually Shows Proof

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Re: I need help on a probability problem

Quote:
 Originally Posted by captain dean my friend and i have been guessing another friends shirt for 22 days. he only wears 5 different shirts, and we have never gotten it right. we are a statistical anomaly. I thought the word problem i gave seemed simpler to explain though, any way, thats interesting. and for the record, the answer i gave in my question on my first post was for 19 days not 22, so i did do the math right. sorry this may not make too much sense as im super tired but hey.... anyway, thanks much
The math assumes your friend is choosing shirts randomly. He may be choosing strategically based on what he thinks you will do. The math also assumes you are guessing randomly, which is basically impossible unless you are using a randomizing mechanism and not just guessing. With a non-random shirt choice, and a non-random "guess" it's possible you will never get it right.

This makes a nice Nash equilibrium example, kind of like rock paper scissors.

Last edited by spadebidder; 12-03-2010 at 09:36 AM.

12-03-2010, 09:42 AM   #10
BruceZ
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Re: I need help on a probability problem

Quote:
 Originally Posted by EvilSteve I assume at least he knows what the five possible colors are, so he's not guessing aqua and chartreuse while the object switches between green red blue orange and purple. Absurd problem otherwise. So, the object changes color each day and stays the same color throughout the day, it might be blue on day 1 then red on day 2, etc? Seems like the hard part here is understanding what the problem is. Could be stated more clearly but I think I understand. Ok so he makes two (different) guesses per day, meaning on any given day 2/5 chance of guessing the correct color for the day. But you want the probability he guesses incorrectly for 22 days straight. On any given day he has a 3/5 chance of not guessing correctly, so 22 straight days of never guessing right is (3/5)^22 = .00001316.
I don't think you took into account that the color randomly changes once a day. That gives you a 1/2 chance of getting it right each day, so your chance of getting it wrong 22 days in a row is (1/2)22 or 1 in 4,194,304.

12-03-2010, 09:43 AM   #11
Actually Shows Proof

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Re: I need help on a probability problem

Quote:
 Originally Posted by BruceZ I don't think you took into account that the color randomly changes once a day. That gives you a 1/2 chance of getting it right each day, so your chance of getting it wrong 22 days in a row is (1/2)22 or 1 in 4,194,304.
I think they just meant he wears a different shirt each day, for the entire day, and they get two guesses (they each guess). So it's 2/5 they get it right on any given day.

12-03-2010, 10:24 AM   #12
BruceZ
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Re: I need help on a probability problem

Quote:
 Originally Posted by spadebidder I think they just meant he wears a different shirt each day, for the entire day, and they get two guesses (they each guess). So it's 2/5 they get it right on any given day.
Yeah, different from the day before. That's what "change" means. You can't wear the same shirt 2 days in a row if you change it every day. It is 2/4 = 1/2 for each day.

12-03-2010, 10:31 AM   #13
Actually Shows Proof

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Re: I need help on a probability problem

Quote:
 Originally Posted by BruceZ Yeah, different from the day before. That's what "change" means. You can't wear the same shirt 2 days in a row if you change it every day. It is 2/4 = 1/2 for each day.
I guess I assumed there was not a condition that the shirt must be different from the day before, because the problem statement originally said a random choice from the 5 available colors.

The OP should clarify that one.

12-03-2010, 10:35 AM   #14
BruceZ
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Re: I need help on a probability problem

Quote:
 Originally Posted by spadebidder I guess I assumed there was not a condition that the shirt must be different from the day before, because the problem statement originally said a random choice from the 5 available colors.
He didn't say a random choice of 5 colors, he said it "randomly changes between 5 different colors once a day". The key is that it must change. Seems clear enough to me.

 12-03-2010, 11:34 AM #15 hepzebah adept   Join Date: Aug 2008 Posts: 714 Re: I need help on a probability problem I will postulate that a guy who only has 5 shirts, probably occasionally wears 'em two days in a row.
 12-03-2010, 11:35 AM #16 hepzebah adept   Join Date: Aug 2008 Posts: 714 Re: I need help on a probability problem Also, if betting on this, I would casually find out the laundry day of the guy. That will give you some predicability.
 12-03-2010, 01:20 PM #17 captain dean stranger   Join Date: Dec 2010 Posts: 5 Re: I need help on a probability problem So if there were 6 t shirts and 14 days there would be a 34% chance? that seems awfully high too me.
12-03-2010, 02:47 PM   #18
cyberfish
journeyman

Join Date: Sep 2004
Posts: 360
Re: I need help on a probability problem

Quote:
 Originally Posted by BruceZ He didn't say a random choice of 5 colors, he said it "randomly changes between 5 different colors once a day". The key is that it must change. Seems clear enough to me.
If your guesses are random between the 5 colors (like OP said) it doesn't matter if the objects color changes or doesn't change at all for 22 days?

What if the color changes at noon and you randomly guess 2 times pre-noon (chance increases to 7/10)?

12-03-2010, 02:53 PM   #19
BruceZ
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Re: I need help on a probability problem

Quote:
 Originally Posted by cyberfish If your guesses are random between the 5 colors (like OP said)
It doesn't say that you guess randomly out of 5 colors. That would be foolish.

Quote:
 What if the color changes at noon and you randomly guess 2 times pre-noon (chance increases to 7/10)?
That's not the problem, but you have to read the whole thread to realize that.

12-03-2010, 04:26 PM   #20
R Gibert

Join Date: Jan 2006
Posts: 936
Re: I need help on a probability problem

Quote:
 Originally Posted by captain dean my friend and i have been guessing another friends shirt for 22 days. he only wears 5 different shirts, and we have never gotten it right. we are a statistical anomaly. I thought the word problem i gave seemed simpler to explain though, any way, thats interesting. and for the record, the answer i gave in my question on my first post was for 19 days not 22, so i did do the math right. sorry this may not make too much sense as im super tired but hey.... anyway, thanks much
Your friends are playing a practical joke on you. They want to see how long you will take for you to get it. Your co-guesser is supplying Mr. Shirts with "inside information." This is a much more likely explanation than guessing wrong every day for 22 days.

 12-03-2010, 04:42 PM #21 EvilSteve Midlife Crisis     Join Date: Mar 2006 Location: 2p2 afterlife Posts: 13,198 Re: I need help on a probability problem

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