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# A student must choose one of the subjects, art, geology, or psychology

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Intern
Joined: 10 Oct 2010
Posts: 6
A student must choose one of the subjects, art, geology, or psychology  [#permalink]

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17 Oct 2010, 08:39
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A student must choose one of the subjects, art, geology, or psychology, as an elective. She is equally likely to choose art or psychology and twice as likely to choose geology. What are the respective probabilities that she chooses art, geology, and psychology?
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Re: A student must choose one of the subjects, art, geology, or psychology  [#permalink]

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17 Oct 2010, 15:18
abeer16 wrote:
A student must choose one of the subjects, art, geology, or psychology, as an
elective. She is equally likely to choose art or psychology and twice as likely
to choose geology. What are the respective probabilities that she chooses art,
geology, and psychology

P(a) = probability of choosing art
P(g) = probability of choosing geology
P(p) = probability of choosing psychology

P(a)=P(p)=P(g)/2

P(a)+P(p)+P(g) = 1 (She must choose one of them, so sum is 1).

Its easy to solve this to get, P(a)=0.25. P(p)=0.25. P(g)=0.5
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Re: A student must choose one of the subjects, art, geology, or psychology  [#permalink]

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18 Oct 2010, 08:46
Let us say the probability that Art is chosen is x.
Then probability that Psychology is chosen is also x (since they are equally likely to be chosen)
The probability that Geology is chosen is 2x since it is twice as likely to be chosen.

Since, she must choose one of them, Probability (Art or Psychology or Geology) = 1 i.e. one subject of the three will definitely get chosen.
Also, she has to choose only one of them, not two or three. Hence Probability (Art or Psychology or Geology) is simply equal to P(Art) + P(Psychology) + P(Geology) (This is the most important step. Since two or more subjects cannot be chosen, there is no overlap in their probabilities. So the probability of choosing any one of them is simply equal to the sum of their individual probabilities. For more clarification, check out the theory of probability.)

1 = x + x + 2x
This gives us x = 1/4 = P(Art) = P(Psychology)
P(Geology) = 2x = 1/2

If all three subjects were equally likely to be chosen, what would be the probability of choosing Art?
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Karishma
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Posts: 783
Re: A student must choose one of the subjects, art, geology, or psychology  [#permalink]

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12 Sep 2018, 05:32
Let us say the probability that Art is chosen is x.
Then probability that Psychology is chosen is also x (since they are equally likely to be chosen)
The probability that Geology is chosen is 2x since it is twice as likely to be chosen.

Since, she must choose one of them, Probability (Art or Psychology or Geology) = 1 i.e. one subject of the three will definitely get chosen.
Also, she has to choose only one of them, not two or three. Hence Probability (Art or Psychology or Geology) is simply equal to P(Art) + P(Psychology) + P(Geology) (This is the most important step. Since two or more subjects cannot be chosen, there is no overlap in their probabilities. So the probability of choosing any one of them is simply equal to the sum of their individual probabilities. For more clarification, check out the theory of probability.)

1 = x + x + 2x
This gives us x = 1/4 = P(Art) = P(Psychology)
P(Geology) = 2x = 1/2

If all three subjects were equally likely to be chosen, what would be the probability of choosing Art?

if each subject had equal likely hood of being selected then x+x+x=1 -> 3x=1 and x=1/3
Is this correct? Thank you.
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- Stne
Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 9230
Location: Pune, India
Re: A student must choose one of the subjects, art, geology, or psychology  [#permalink]

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13 Sep 2018, 05:28
1
stne wrote:
Let us say the probability that Art is chosen is x.
Then probability that Psychology is chosen is also x (since they are equally likely to be chosen)
The probability that Geology is chosen is 2x since it is twice as likely to be chosen.

Since, she must choose one of them, Probability (Art or Psychology or Geology) = 1 i.e. one subject of the three will definitely get chosen.
Also, she has to choose only one of them, not two or three. Hence Probability (Art or Psychology or Geology) is simply equal to P(Art) + P(Psychology) + P(Geology) (This is the most important step. Since two or more subjects cannot be chosen, there is no overlap in their probabilities. So the probability of choosing any one of them is simply equal to the sum of their individual probabilities. For more clarification, check out the theory of probability.)

1 = x + x + 2x
This gives us x = 1/4 = P(Art) = P(Psychology)
P(Geology) = 2x = 1/2

If all three subjects were equally likely to be chosen, what would be the probability of choosing Art?

if each subject had equal likely hood of being selected then x+x+x=1 -> 3x=1 and x=1/3
Is this correct? Thank you.

Yes absolutely! Equal probability of one of the three gives 1/3 for each.
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Karishma
Veritas Prep GMAT Instructor

Re: A student must choose one of the subjects, art, geology, or psychology   [#permalink] 13 Sep 2018, 05:28
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