Dice Probability
I'm a CJ/CIS major working in a Math department, and I've been asked to convert an open ended question test to multiple choice test. I found your site "The Wizard of Odds", and was directed here to ask my question. What is the probability of rolling a 2 given your roll only being even, when rolling a 20 sided die? Thank you for your help.
-R
Quote: rickwolves91Hey Wizard,
I'm a CJ/CIS major working in a Math department, and I've been asked to convert an open ended question test to multiple choice test. I found your site "The Wizard of Odds", and was directed here to ask my question. What is the probability of rolling a 2 given your roll only being even, when rolling a 20 sided die? Thank you for your help.
-R
I know what you probably meant, but you actually need to specify what's on the 20-sided die. If it's 10 through 200 in increments of 10, the probability is zero.
Under the assumption that the d20 contains integers 1..20, here are the answers I'd list on a four-choice pick:
a) 1/2
b) 1/5
c) 1/10
d) 1/20
Edit: What's a "CJ" major?
Quote: MathExtremistI know what you probably meant, but you actually need to specify what's on the 20-sided die. If it's 10 through 200 in increments of 10, the probability is zero.
Under the assumption that the d20 contains integers 1..20, here are the answers I'd list on a four-choice pick:
a) 1/2
b) 1/5
c) 1/10
d) 1/20
Edit: What's a "CJ" major?
Funny, I read it differently for the same reason. I "decided" the question stipulated the 20 sides were numbered 2 thru 20, evens only, with 2 identical sides per number, before I read ME's answer. Be interested to see the clarification.
I'm guessing "Criminal Justice/Crime Scene Investigation" for the major.
I played some Dungeons & Dragons as a kid, so I've seen a d20 or two.
With a d20 (integers 1 through 20) the odds of rolling a 2 would be (drum roll please)... 1 in 20.
If you're saying "I've got a d20. The next roll will be an even number. What are the odds of that number being a 2?" Then I'd have to say the odds are 1 in 10. But how does one guarantee that no odd number will be rolled?
I'm thinking CJ/CIS is Criminal Justice/Criminal Information Systems.
Quote: TerribleTomIf you're saying "I've got a d20. The next roll will be an even number. What are the odds of that number being a 2?" Then I'd have to say the odds are 1 in 10. But how does one guarantee that no odd number will be rolled?
It's a conditional probability problem. It's guaranteed because the question says it's guaranteed. It's a math question, not a physics question.
My HS world geography teacher was a sadistic guy. He put patterns into all his tests, but he also rotated the correct answers. So my 100% score could go a,b,c,d,a,b,c,d... while my friend could have the same answers but need to respond d,c,b,a,d,c,b,a... Made colluding and/or selling answers a real pain.
I'm sorry, fellas. The numbers on the dice are 1-20. My understanding of this questions is, what is the probability you will roll an even number and what is the probability that even number will be 2? The next question on the test is "Find the probability of drawing a king given that you draw a spade." I'm pretty sure both questions require the same method to solve.
Quote: rickwolves91CJ - Criminal Justice
The numbers on the 20 sided dice are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20. If this reply sounds smartalically, I appologize. I don't really understand what you mean needing to "specify what's on the 20-sided die." Anyhow, which is the answer; a, b, c, or d?
There are 10 even numbers. If you know for sure that it is even, then the answer is 1/10.
You can also solve this by going through the formula for conditional probability. To be honest I always have to look it up (check wikipedia for conditional probability), but you will end up dividing 1/20 by 10/20 and get 1/10.
"But how does one guarantee that no odd number will be rolled?" Probable probability.
Thank you.
Quote: rickwolves91"But how does one guarantee that no odd number will be rolled?" Probable probability.
An even number being rolled is the condition of the problem. i.e i tell you i rolled an even number. you would then know that the probability of the number being a 2 is 0.1.
Quote: AxiomOfChoiceIt's a conditional probability problem. It's guaranteed because the question says it's guaranteed. It's a math question, not a physics question.
I get that. The initial question was phrased very oddly.
A better phrasing of the question might be "A d20 was rolled and the result was an even number. What are the odds that the result was a 2?"
Quote:The initial question was phrased very oddly.
it might be oddly phrased out of context.. but it probably comes out of a textbook right after the chapter on conditional probability.. the phrase "given" is how a statistician would describe it.. the lingo isn't always how people would describe things in plain English.
Quote: rickwolves91The next question on the test is "Find the probability of drawing a king given that you draw a spade." I'm pretty sure both questions require the same method to solve.
You're correct. Same method for this. Conditional probability, as mentioned above. Given that you've drawn a spade, you have 13 cards to pick from, one of which is the King. Therefore, the answer to your question is 1/13
