Probability: Die roll until first number

In summary, the problem asks to find the probability that a fair die is rolled until a '5' appears, and to determine the likelihood that the number of rolls is divisible by 3. To approach this, the probabilities of each possible number of rolls can be calculated and summed to find the overall probability.
  • #1
stosw
21
0

Homework Statement



Roll a fair die until the first time you roll a '5' then stop. Let X be the number of times you rolled the die. What is the probability that X is divisible by 3?

Homework Equations



E[X] ?

The Attempt at a Solution



I honestly have no idea how to even approach this sort of thing. My first thought was either a bionomial or geometric series, but that could possibly be endless.

any sort of hints or suggestions would be great.
 
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  • #2
Let's first start by figuring out the probabilities involved.

Can you find out [itex]P\{X=1\}[/itex]? (that is: the probability your first throw is a 5). What about [itex]P\{X=2\}[/itex]? (the probability that your first throw is not a five, but your second throw is). Can you find a general formula for [itex]P\{X=n\}[/itex]?
 
  • #3
Find [itex]P(X = x)[/itex]. [itex]X[/itex] is divisible by [itex]3[/itex] if and only if [itex]X = 3k[/itex] for some integer [itex]k[/itex]. Therefore, the probability that [itex]X[/itex] is divisible by [itex]3[/itex] is given by [itex]\sum_{k=1}^{\infty} P(X = 3k)[/itex].
 

1. What is "Probability: Die roll until first number"?

"Probability: Die roll until first number" is a statistical concept that calculates the likelihood of rolling a specific number on a die after multiple rolls. It assumes that the die is rolled repeatedly until a specific number is rolled for the first time, and calculates the probability of this event occurring.

2. How is the probability of "Probability: Die roll until first number" calculated?

The probability of "Probability: Die roll until first number" is calculated by dividing the number of possible outcomes where the desired number is rolled for the first time by the total number of possible outcomes. For example, if you are rolling a standard six-sided die and want to know the probability of rolling a 3 for the first time, the probability would be 1/6 or 16.67%.

3. What is the relationship between the number of rolls and the probability of "Probability: Die roll until first number"?

The more rolls that are performed, the higher the probability of rolling the desired number for the first time. This is because with each roll, the number of possible outcomes decreases, making it more likely for the desired number to be rolled. However, the overall probability will never reach 100% as there is always a chance that the desired number will never be rolled.

4. How does the number of sides on the die affect the probability of "Probability: Die roll until first number"?

The number of sides on the die directly affects the probability of "Probability: Die roll until first number". As the number of sides increases, the probability of rolling a specific number for the first time decreases. For example, the probability of rolling a 3 for the first time on a 6-sided die is 1/6, but on a 12-sided die it would be 1/12.

5. How is "Probability: Die roll until first number" used in real-world scenarios?

"Probability: Die roll until first number" can be applied to various real-world scenarios, such as predicting the likelihood of winning a game of chance or determining the probability of a certain outcome in a series of events. It can also be used in fields such as finance and economics to make educated decisions based on probabilities and potential risks.

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