A bag contains 3 white balls, 3 black balls & 2 red balls : Problem Solving (PS)

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A bag contains 3 white balls, 3 black balls & 2 red balls

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Director

Joined: 23 Jan 2013

Posts: 560

Schools: Cambridge'16

Re: A bag contains 3 white balls, 3 black balls & 2 red balls [#permalink]

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02 Aug 2015, 04:22

First option (first two balls are not red and the last is red): 6/8*5/7*2/6=5/28

Second option (first or second ball is red and the last is red): (6/8*2/7*1/6)*2=2/28

So, 5/28+2/28=7/28=1/4

A

Intern

Joined: 07 Jan 2016

Posts: 23

Schools: AGSM '18

Re: A bag contains 3 white balls, 3 black balls & 2 red balls [#permalink]

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13 Mar 2016, 06:02

Could you tell me more about your way to give that formula? Thank you.

zisis wrote:

Bunuel wrote:

krishan wrote:

A bag contains 3 white balls, 3 black balls & 2 red balls. One by one three balls are drawn out without replacement. What is the probability that the third ball is red?

A. 0.25
B. 0.15
C. 0.35
D. 0.45
E. 0.40

Thanks

The initial probability of drawing red ball is 2/8. Without knowing the other results, the probability of drawing red ball will not change for ANY successive drawing: second, third, fourth...

Similar problems:
ps-probability-gmat-made-me-lose-my-marbles-36830.html?hilit=initial%20draw
probability-of-picking-last-97015.html?hilit=initial%20probability
probability-question-please-solve-90272.html?hilit=initial%20probability#p686028

Hope it's clear.

!!!!!
went the long way - still under 2min

RedOtherRed +OtherRR+OOR

\(\frac{1}{28}+\frac{1}{28}+\frac{5}{28}=\frac{7}{28}=\frac{1}{4}\)

Non-Human User

Joined: 09 Sep 2013

Posts: 9461

Re: A bag contains 3 white balls, 3 black balls & 2 red balls [#permalink]

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24 May 2018, 16:00

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