marbles in a bag: probability that A draws black first?

tsh44

Junior Member
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Sep 4, 2005
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Hi I am not getting the logic of this problem.

A bag contains 4 red marbles and 1 black marble. Two persons, A & B, take turns drawing a marble from the bag without replacement. The first person to draw a black marble wins. What is the probability that person A, who draws first wins?

The answer is 3/5 according to the text. But how do you arrive at this answer?
 
Re: marbles

tsh44 said:
Hi I am not getting the logic of this problem.

A bag contains 4 red marbles and 1 black marble. Two persons, A & B, take turns drawing a marble from the bag without replacement. The first person to draw a black marble wins. What is the probability that person A, who draws first wins?

The answer is 3/5 according to the text. But how do you arrive at this answer?
What are the possible outcomes?

brrrr
rbrrr
rrbrr
rrrbr
rrrrb

For which ones does A win?
 
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