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 Probability and Statistics Basic Probability and Statistics Math Forum

 February 25th, 2019, 09:12 PM #1 Member   Joined: Aug 2018 From: România Posts: 90 Thanks: 6 Probabilities Hello, In a box there are $\displaystyle 25$ white balls and $\displaystyle 15$ red balls.After each extraction, the ball is returned to box. 1) What is the probability of $\displaystyle 3$ white balls coming out consecutively after $\displaystyle 5$ extractions? 2) What is the probability that $\displaystyle 2$ white balls and $\displaystyle 1$ red ball coming out consecutively after $\displaystyle 10$ extractions? 3) How many extractions should be made so that the probability of point 2) is greater than $\displaystyle \frac{1}{2}$? All the best, Integrator Last edited by Integrator; February 25th, 2019 at 09:15 PM. February 26th, 2019, 11:22 AM #2 Senior Member   Joined: Sep 2015 From: USA Posts: 2,590 Thanks: 1434 1) $p = \dfrac{25}{40}=\dfrac{5}{8}$ $1-p = \dfrac{3}{8}$ $P[\text{3 consecutive white balls}] = \sum \limits_{w=3}^5 P[\text{3 consecutive white balls}|w \text{ white balls chosen}]$ $P[3c|3] = 3\cdot p^3 (1-p)^2$ $P[3c|4] = 4 \cdot 2 p^4 (1-p)$ $P[3c|5] = p^5$ $P[\text{3 consecutive white balls}] = 3p^3 (1-p)^2+4p^4 (1-p)+p^5$ I leave it to you to crunch the numbers. March 4th, 2019, 08:52 PM   #3
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Quote:
 Originally Posted by romsek 1) $p = \dfrac{25}{40}=\dfrac{5}{8}$ $1-p = \dfrac{3}{8}$ $P[\text{3 consecutive white balls}] = \sum \limits_{w=3}^5 P[\text{3 consecutive white balls}|w \text{ white balls chosen}]$ $P[3c|3] = 3\cdot p^3 (1-p)^2$ $P[3c|4] = 4 \cdot 2 p^4 (1-p)$ $P[3c|5] = p^5$ $P[\text{3 consecutive white balls}] = 3p^3 (1-p)^2+4p^4 (1-p)+p^5$ I leave it to you to crunch the numbers.
Hello,

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It does not have to be $\displaystyle P[3c|4] = 4 \cdot p^4 (1-p)$?
It does not have to be $\displaystyle P[3c|5] = 5p^5$?
Thank you very much!

All the best,

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