Hello!
I am puzzled by the answer of a probability question which I got from one of the GMAT Prep Now videos.
Question: If a teacher randomly selects 2 students from a group of 4 boys and 4 girls, what is the probability that the selection contains at least 1 boy?
I know that there are 2 methods to solve this question:
1) P(at least 1 boy) = 1 - P(0 boys)
= 1- P(all girls)
= 1 - P(girl 1 AND girl 2)
= 1 - (4/8*3/7) = 1 - 3/14
= 11/14 [correct answer]
However, when I try to solve it using the other method (as below), I can't seem to get the same answer.
2) P(at least 1 boy) = P(1 boy or 2 boys)
= P(1 boy) + P(2 boys)
= 4/8 + (4/8*3/7)
= 1/2 + 3/14
= 10/14
I would appreciate someone pointing out to me where I went wrong. Was it a calculation mistake or a conceptual mistake?
Thank you very much!
Probability Puzzle
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- theCodeToGMAT
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CASE 2:
P(at least 1 boy) = P(1 boy or 2 boys)
= P(1 boy & 1 girl) + P(2 boys & no girl) = 4C1*4C1/8C2 + 4C2/8C2 = 4/7 + 3/14 = 11/14
P(at least 1 boy) = P(1 boy or 2 boys)
= P(1 boy & 1 girl) + P(2 boys & no girl) = 4C1*4C1/8C2 + 4C2/8C2 = 4/7 + 3/14 = 11/14
R A H U L
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cvj0873 wrote:
2) P(at least 1 boy) = P(1 boy or 2 boys)
= P(1 boy) + P(2 boys) ----> You forgot to consider 1 GIRL
= 4/8 + (4/8*3/7)
= 1/2 + 3/14
= 10/14
R A H U L
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Hi cvj0873
If you're going to answer this question with the second method that you listed, you have to add a few more details to your work:
Probability of 1 boy and 1 girl:
2 possibilities:
BG = (4/8)(4/7) = 16/56
OR
GB = (4/8)(4/7) = 16/56
So, the TOTAL probability of 1 boy and 1 girl = 32/56 NOT 1/2
When combined with the probability of getting 2 boys:
(4/8)(3/7) = 12/56
The GRAND TOTAL = 32/56 + 12/56 = 44/56 = 11/14
GMAT assassins aren't born, they're made,
Rich
If you're going to answer this question with the second method that you listed, you have to add a few more details to your work:
Probability of 1 boy and 1 girl:
2 possibilities:
BG = (4/8)(4/7) = 16/56
OR
GB = (4/8)(4/7) = 16/56
So, the TOTAL probability of 1 boy and 1 girl = 32/56 NOT 1/2
When combined with the probability of getting 2 boys:
(4/8)(3/7) = 12/56
The GRAND TOTAL = 32/56 + 12/56 = 44/56 = 11/14
GMAT assassins aren't born, they're made,
Rich
GMAT/MBA Expert
- lunarpower
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If you are not 100% sure about calculating, just make a list. Let's say A, B, C, D are boys and e, f, g, h are girls.
AB AC AD Ae Af Ag Ah
BC BD Be Bf Bg Bh
CD Ce Cf Cg Ch
De Df Dg Dh
ef eg eh
fg fh
gh
There are 28 things in the list. The red ones have at least one boy.
There are 6 things that are not red, so there are 28 - 6 = 22 red things.
So, the probability is 22/28 = 11/14.
It took me about 30 seconds to make that list, and people here are generally a lot faster than I am. So this method is also very efficient.
AB AC AD Ae Af Ag Ah
BC BD Be Bf Bg Bh
CD Ce Cf Cg Ch
De Df Dg Dh
ef eg eh
fg fh
gh
There are 28 things in the list. The red ones have at least one boy.
There are 6 things that are not red, so there are 28 - 6 = 22 red things.
So, the probability is 22/28 = 11/14.
It took me about 30 seconds to make that list, and people here are generally a lot faster than I am. So this method is also very efficient.
Ron has been teaching various standardized tests for 20 years.
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Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi
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Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.
Yves Saint-Laurent
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Learn more about ron