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No math is needed here if we understand the following concept:sachindia wrote:A basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
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3/8
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5/8
11/16
P(blue on the 3rd pick) = P(blue on the 1st pick) = 5/8.A basket contains 3 white and 5 blue balls. Mary will extract TWO BALLS at random and keep them. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
hey , thanks a lot mitch for this. Does this work for problems with and without replacement?GMATGuruNY wrote:No math is needed here if we understand the following concept:sachindia wrote:A basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
1/3
3/8
1/2
5/8
11/16
The probability of selecting X on the NTH pick is equal to the probability of selecting X on the FIRST pick.
Thus, P(blue on the 2nd pick) = P(blue on the first pick) = 5/8.
The correct answer is D.
The answer would be the same even if the problem were as follows:
P(blue on the 3rd pick) = P(blue on the 1st pick) = 5/8.A basket contains 3 white and 5 blue balls. Mary will extract TWO BALLS at random and keep them. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
Other problems that test the same concept:
https://www.beatthegmat.com/probablity-ques-t60161.html
https://www.beatthegmat.com/manhattan-pr ... 89481.html (2 posts)
https://www.beatthegmat.com/a-box-contai ... 51368.html
Yes.sachindia wrote:hey , thanks a lot mitch for this. Does this work for problems with and without replacement?GMATGuruNY wrote:No math is needed here if we understand the following concept:sachindia wrote:A basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
1/3
3/8
1/2
5/8
11/16
The probability of selecting X on the NTH pick is equal to the probability of selecting X on the FIRST pick.
Thus, P(blue on the 2nd pick) = P(blue on the first pick) = 5/8.
The correct answer is D.
The answer would be the same even if the problem were as follows:
P(blue on the 3rd pick) = P(blue on the 1st pick) = 5/8.A basket contains 3 white and 5 blue balls. Mary will extract TWO BALLS at random and keep them. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
Other problems that test the same concept:
https://www.beatthegmat.com/probablity-ques-t60161.html
https://www.beatthegmat.com/manhattan-pr ... 89481.html (2 posts)
https://www.beatthegmat.com/a-box-contai ... 51368.html
This question reminds me of my childhood, when my friends and I would sometimes "draw straws" to randomly select one person to do something (often either work, like getting wood for the fire, or dumb, like eating something that shouldn't be eaten).sachindia wrote:A basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
1/3
3/8
1/2
5/8
11/16
This is interesting. Where does this concept come from? I've not seen it used anywhere else. Thanks.GMATGuruNY wrote:No math is needed here if we understand the following concept:sachindia wrote:A basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
1/3
3/8
1/2
5/8
11/16
The probability of selecting X on the NTH pick is equal to the probability of selecting X on the FIRST pick.
Thus, P(blue on the 2nd pick) = P(blue on the first pick) = 5/8.
The correct answer is D.
The answer would be the same even if the problem were as follows:
P(blue on the 3rd pick) = P(blue on the 1st pick) = 5/8.A basket contains 3 white and 5 blue balls. Mary will extract TWO BALLS at random and keep them. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
Other problems that test the same concept:
https://www.beatthegmat.com/probablity-ques-t60161.html
https://www.beatthegmat.com/manhattan-pr ... 89481.html (2 posts)
https://www.beatthegmat.com/a-box-contai ... 51368.html
But Mary keeps the ball...How is it the same if she didn't keep it? Thank you.Brent@GMATPrepNow wrote:This question reminds me of my childhood, when my friends and I would sometimes "draw straws" to randomly select one person to do something (often either work, like getting wood for the fire, or dumb, like eating something that shouldn't be eaten).sachindia wrote:A basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
1/3
3/8
1/2
5/8
11/16
So, someone would hold up n pieces of grass (for n guys), and one of those pieces was very short. The person who selected the shortest piece was the one who had to perform the task.
There was always one guy who wanted to choose his piece last. His reasoning was that his chances of drawing the shortest piece were minimized since every person before him had a chance of drawing the short piece before it got to his turn.
The truth of the matter is that each of the n guys had a 1/n chance of selecting the shortest piece, regardless of the order in which they selected.
The same applies to the original question here. 5 of the 8 balls are blue, so P(John selects a blue ball) = 5/8
Cheers,
Brent
Let's solve this question a different way.sachindia wrote:A basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. If, after that, John will extract one ball at random, what is the probability that John will extract a blue ball?
1/3
3/8
1/2
5/8
11/16
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