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There is a 50% chance Jen will visit Chile this year, while there is a

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There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 23 Sep 2015, 08:58
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There is a 50% chance Jen will visit Chile this year, while there is a 25% chance that she will visit Madagascar this year. What is the probability that Jen will visit either Chile or Madagascar this year, but NOT both?[/b]

A. 25%
B. 50%
C. 62.5%
D. 63.5%
E. 75%

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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 23 Sep 2015, 23:01
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Bunuel wrote:
There is a 50% chance Jen will visit Chile this year, while there is a 25% chance that she will visit Madagascar this year. What is the probability that Jen will visit either Chile or Madagascar this year, but NOT both?[/b]

A. 25%
B. 50%
C. 62.5%
D. 63.5%
E. 75%



This question was written to help people understand the use of sets in probability. Think of the venn diagram with two overlapping sets.

n(A or B) = n(A) + n(B) - n(A and B) ........(I)
(We subtract n(A and B) because it is double counted. The overlapping part was counted twice, once in each circle so it is subtracted out.)

n(A or B but not Both) = n(A) + n(B) - n(A and B) - n(A and B) ............(II)
(We subtract another n(A and B) so that 'Both' is subtracted out. The overlapping part was counted once in (I); now it is totally removed)

P (visit C or M but not Both) = 1/2 + 1/4 - (1/2*1/4) - (1/2*1/4) = 1/2

Answer (B)
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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 23 Sep 2015, 10:43
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Bunuel wrote:
There is a 50% chance Jen will visit Chile this year, while there is a 25% chance that she will visit Madagascar this year. What is the probability that Jen will visit either Chile or Madagascar this year, but NOT both?[/b]

A. 25%
B. 50%
C. 62.5%
D. 63.5%
E. 75%


Our desired probability would be
(Probability of visiting Chile) * (Probability of not visiting Madagascar) + (Probability of not visiting Chile) * (Probability of visiting Madagascar)
= \(\frac{1}{2}*\frac{3}{4} + \frac{1}{2}*\frac{1}{4}\) \(=\frac{4}{8}\) \(=\frac{1}{2}\)
= 50 %

Answer:-B
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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 23 Sep 2015, 09:14
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Bunuel wrote:
There is a 50% chance Jen will visit Chile this year, while there is a 25% chance that she will visit Madagascar this year. What is the probability that Jen will visit either Chile or Madagascar this year, but NOT both?[/b]

A. 25%
B. 50%
C. 62.5%
D. 63.5%
E. 75%


\(\frac{1}{2}+\frac{1}{4}-\frac{1}{2}*\frac{1}{4}*2\)=\(\frac{1}{2}\)..
B
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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 23 Sep 2015, 09:42
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The thing to note in the question is she would not be visiting both.

Thus p(Chile)*(1-p(Madagascar))+p(Madagascar)*(1-p(Chile))
which is
(1/2*3/4) + (1/4*1/2),
3/8*1/8=1/2
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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 23 Sep 2015, 12:48
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1 - (the probability of choosing neither) - (probability of choosing both)

Probability of choosing neither = 1/2 * 3/4 = 3/8
Probability of choosing both = 1/2 * 1/4 = 1/8

1 - 3/8 - 1/8 = 4/8 or 1/2

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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 27 Sep 2015, 11:05
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Bunuel wrote:
There is a 50% chance Jen will visit Chile this year, while there is a 25% chance that she will visit Madagascar this year. What is the probability that Jen will visit either Chile or Madagascar this year, but NOT both?[/b]

A. 25%
B. 50%
C. 62.5%
D. 63.5%
E. 75%


VERITAS PREP OFFICIAL SOLUTION:

Putting aside her peculiar travel schedule for a moment, (visiting Western South America and South East Africa without stopping by the Falklands?) what is the probability that Jen travels to one of these locales but not the other? Dutifully applying our memorized probability formula: the probability of visiting Chile is 0.5, and the probability of visiting Madagascar is 0.25, we get 0.5 + 0.25 – (0.5*0.25) or 0.75 – 0.125 = 0.625. Or answer choice C. (As Admiral Ackbar tried to warn us in 1983: It’s a Trap!)

But why doesn’t the formula yield the correct choice? The answer lies in the question stem. This particular question asks us the probability of visiting Chile or Madagascar, but not both. The formula gives us the probability of visiting either, implicitly allowing the choice of visiting both. The probability of visiting either will indeed be 62.5% (or 5/8), but this is the probability of visiting Chile, Madagascar, or both. Verifying the converse, the probability of visiting neither is P(¬Chile) and P(¬Madagascar), or 0.5 * 0.75 = 0.375 (or 3/8), confirming that our 0.625 is merely the probability of not staying home this year.

The obvious question now is: Why doesn’t the formula work? Didn’t the formula already account for the possibility of both? How do I solve this question correctly? (Admittedly, these are three questions and not one). The key to answering all of them is the same, though. Let’s go through the logic of the formula P(C) + P(M) – P(C&M):

The first argument allows for all possibilities of visiting Chile, regardless of what happens with Madagascar.

The second argument allows for all possibilities of visiting Madagascar, regardless of what happens with Chile (or the Falklands)

The third argument is the possibility of both occurring.

The formula works because P(C) accounts for the both choice, and P(M) accounts for the both choice as well, indicating that this option has been double counted. In order to count it only once, we need to remove one instance of it. This is why the formula works and is popular; it addresses the inherent problem in probability, double counting certain situations. The same logic applies to Venn diagrams and other similar question types where counting the same argument twice (or thrice) can occur.

In practice, this question could then be solved using the default formula of P(C) + P(M) – P(C&M) and then subtracting P(C&M) again… or simply P(C) + P(M) – 2*P(C&M). Simply put, the first two arguments in the formula account for the “both” possibility twice, so we must remove it twice to answer the question at hand.
A somewhat similar yet more straight forward alternative is to go bottom up instead of top down. The probability of going only to Chile is P(C) * P(¬M) = 0.5 * 0.75 = 0.375. The probability of going only to Madagascar is P(M) * P(¬C) = 0.25 * 0.5 = 0.125. Adding those two probabilities together yields the correct answer of 0.5 or 50%. The revised formula also yields this result (0.5 + 0.25 – 2 (0.125) = 0.5), so we can feel confident in our answer choice and not any of the other tempting answer choices.

The answer is (B).
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There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post Updated on: 19 Jun 2016, 06:54
I have a question about the P(A or B) formula.

So does this mean that by using the P(A) + P(B) - P(A and B) we are not subtracting the overlap to get the "pure" "A or B and not A and B at the same time" but rather we are simply correcting a mathematical error that otherwise would have occurred if we didn't subtract the overlap of "A and B" to get "A or B"?

Another observation is that in quotidian language when we say "A or B" we mean "or A, or B, but not both". eg: "I will have wine or beer" so here I want to say that I will have beer or wine, but not both. So I am implicitly excluding the possibility of having both.

However in the context of probability, I would have to be more explicit if wanted to say "A or B, but not both"

So by simply saying "A or B" in the context of math, we are still leaving in the possibility of having both A and B in contrast of saying the same thing in the quotidian language. To completely eliminate the possibility of "A and B" happening, we would have to first make the correct mathematical calculation by P(A) + P(B) - P(A and B) to get the "A or B and maybe both" expression and then subtract the overlap ones again to actually remove it for good and get the "isolated" "A or B but not both".

Am I correct here? And it seems that after so many years I finally understood the meaning of the P(A) + P(B) - P(A and B) formula...

Another question: is the P(A) + P(B) - P(A and B) and "At least one" so 1- P(of what we want to happen not happening) the exact same thing?

Thank you!

Originally posted by iliavko on 19 Jun 2016, 06:24.
Last edited by iliavko on 19 Jun 2016, 06:54, edited 2 times in total.
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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 19 Jun 2016, 06:51
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There are 4 total possibilities: (M,C) ; (M) ;(C) ;(No visit to any of two). Now Jen visiting M or C but not both is possible in only 2 of the 4 scenario. So the chances are 50%.
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There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 19 Jun 2016, 07:23
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iliavko wrote:
I have a question about the P(A or B) formula.

So does this mean that by using the P(A) + P(B) - P(A and B) we are not subtracting the overlap to get the "pure" "A or B and not A and B at the same time" but rather we are simply correcting a mathematical error that otherwise would have occurred if we didn't subtract the overlap of "A and B" to get "A or B"?

Another observation is that in quotidian language when we say "A or B" we mean "or A, or B, but not both". eg: "I will have wine or beer" so here I want to say that I will have beer or wine, but not both. So I am implicitly excluding the possibility of having both.

However in the context of probability, I would have to be more explicit if wanted to say "A or B, but not both"

So by simply saying "A or B" in the context of math, we are still leaving in the possibility of having both A and B in contrast of saying the same thing in the quotidian language. To completely eliminate the possibility of "A and B" happening, we would have to first make the correct mathematical calculation by P(A) + P(B) - P(A and B) to get the "A or B and maybe both" expression and then subtract the overlap ones again to actually remove it for good and get the "isolated" "A or B but not both".

Am I correct here? And it seems that after so many years I finally understood the meaning of the P(A) + P(B) - P(A and B) formula...

Another question: is the P(A) + P(B) - P(A and B) and "At least one" so 1- P(of what we want to happen not happening) the exact same thing?

Thank you!


Hi,

Remember that the OVERLAP is part of both P(A) and P(B)....
so for removing the repetition, we subtract P(A and B) once...

this gives us P(A) or P(B)... ofcourse this will include the overlap..

when you have to completely remove the overlap..
remove it from P(A) and P(B)
,
so P(A) - P(A and B) + P(B) - P(A and B) = P(A) + P(B) - 2P(A and B).....
so our answer becomes P(A) + P(B) - 2P(A and B) = \(0.5+0.25 - 2(0.5*0.25) = 0.75 - 2(0.125) = 0.75 - 0.25 = 0.5\)... or 50%

Hope it helps you
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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 19 Jun 2016, 09:37
Chetan,

Thank you so much for the clarification! I was confused with the meaning of the formula..

Cheers!
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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 20 Aug 2018, 13:08
kunal555 wrote:
Bunuel wrote:
There is a 50% chance Jen will visit Chile this year, while there is a 25% chance that she will visit Madagascar this year. What is the probability that Jen will visit either Chile or Madagascar this year, but NOT both?[/b]

A. 25%
B. 50%
C. 62.5%
D. 63.5%
E. 75%


Our desired probability would be
(Probability of visiting Chile) * (Probability of not visiting Madagascar) + (Probability of not visiting Chile) * (Probability of visiting Madagascar)
= \(\frac{1}{2}*\frac{3}{4} + \frac{1}{2}*\frac{1}{4}\) \(=\frac{4}{8}\) \(=\frac{1}{2}\)
= 50 %

Answer:-B


This explanation really made sense for me!
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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 23 Aug 2018, 16:36
Bunuel wrote:
There is a 50% chance Jen will visit Chile this year, while there is a 25% chance that she will visit Madagascar this year. What is the probability that Jen will visit either Chile or Madagascar this year, but NOT both?[/b]

A. 25%
B. 50%
C. 62.5%
D. 63.5%
E. 75%


There are two different scenarios that satisfy the requirement:

(1) She visits Chile but NOT Madagascar. The probability of this event is 1/2 x 3/4 = 3/8.

(2) She doesn’t visit Chile but she DOES visit Madagascar. The probability is 1/2 x 1/4 = 1/8.

We add the probabilities, obtaining 3/8 + 1/8 = 4/8 = ½, or 50%.

Answer: B
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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 05 Sep 2018, 08:37
A tip: Use Venn diagrams in this type of questions. Your life will become much easier.
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Re: There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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New post 16 Sep 2018, 20:02
Hi Hero8888, would you please draw the Venn diagram and suggest the logic? I want to ensure that I understood the point. Thank you

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There is a 50% chance Jen will visit Chile this year, while there is a  [#permalink]

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