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# A box contains 20 balls: 10 white and 10 black. The balls can be drawn

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Math Expert
Joined: 02 Sep 2009
Posts: 52902
A box contains 20 balls: 10 white and 10 black. The balls can be drawn  [#permalink]

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02 Oct 2018, 01:28
00:00

Difficulty:

15% (low)

Question Stats:

86% (01:56) correct 14% (02:30) wrong based on 49 sessions

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A box contains 20 balls: 10 white and 10 black. The balls can be drawn at random. If the first three drawn balls are black, what is the probability that the next two drawn balls will also be black?

(A) 21/136
(B) 4/17
(C) 1/3
(D) 4/7
(E) 2/5

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Joined: 20 Feb 2015
Posts: 795
Concentration: Strategy, General Management
Re: A box contains 20 balls: 10 white and 10 black. The balls can be drawn  [#permalink]

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02 Oct 2018, 05:47
Bunuel wrote:
A box contains 20 balls: 10 white and 10 black. The balls can be drawn at random. If the first three drawn balls are black, what is the probability that the next two drawn balls will also be black?

(A) 21/136
(B) 4/17
(C) 1/3
(D) 4/7
(E) 2/5

3 black balls are already drawn , remaining are 7 black and 10 white balls
number of ways we can draw two black balls from remaining = $$7c2$$
total number of ways we can draw two balls = $$17c2$$
required probability = $$7c2$$ / $$17c2$$ = 21/136
Re: A box contains 20 balls: 10 white and 10 black. The balls can be drawn   [#permalink] 02 Oct 2018, 05:47
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# A box contains 20 balls: 10 white and 10 black. The balls can be drawn

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