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What is the probability of picking a blue ball...

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What is the probability of picking a blue ball...

by AAPL » Tue Oct 31, 2017 9:59 am
What is the probability of picking a blue ball in 2nd pick from a bag containing only blue and red ball, if the first ball picked was red?

(1) the number of red ball is one more than the number of blue ball.
(2) the number of red ball is 7.

The OA is A.

I don't have it clear. Please, can any expert explain this DS question for me? Thanks.
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Source: — Data Sufficiency |
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What is the probability of picking a blue ball

by MartyMurray » Fri Nov 03, 2017 11:23 pm
What is the probability of picking a blue ball in 2nd pick from a bag containing only blue and red balls, if the first ball picked was red?

(1) the number of red balls is one more than the number of blue balls.
(2) the number of red ball is 7.

Statement 1:

Consider the following.

If the number of red balls is one greater than the number of blue balls, then after one red ball is removed, the number of red balls will be equal to the number of blue balls.

At that point, no matter how many red balls and blue balls there are in the bag, the probability of choosing a blue ball will be 0.50.

Sufficient.

Statement 2:

If the number of red balls is 7, then after one is removed, the number of red balls will be 6.

That information does not tell us either the number of blue balls or the ratio of red balls to blue balls. Without one of those facts, there is no way to determine the probability of choosing a blue ball.

Insufficient.

The correct answer is A.
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