Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 500,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

There is a 50% chance Walcir will visit Chile this year [#permalink]

Show Tags

25 Feb 2013, 02:23

1

This post received KUDOS

6

This post was BOOKMARKED

00:00

A

B

C

D

E

Difficulty:

65% (hard)

Question Stats:

49% (02:22) correct
51% (01:05) wrong based on 192 sessions

HideShow timer Statistics

There is a 50% chance Walcir will visit Chile this year, while there is a 25% chance that he will visit Madagascar this year. What is the probability that Walcir will visit either Chile or Madagascar this year, but NOT both?

There is a 50% chance Walcir will visit Chile this year, while there is a 25% chance that he will visit Madagascar this year. What is the probability that Walcir will visit either Chile or Madagascar this year, but NOT both?

25%

50%

62.5%

63.5%

75%

50/50 chances are fun because the probability of getting either answer is the same.

There are four basic possibilities here: C & M, C & ¬M, ¬C & M, ¬C & ¬M. These events are independent, so the probability of each is a straight multiplication.

C & ¬M = 0.5 * 0.75 = 0.375 ¬C & M = 0.5 * 0.25 = 0.125 Add these two up and get 0.500. Or 50%.

For completion's sake, the other two probabilities are:

C & M = 0.5 * 0.25 = 0.125 ¬C & ¬M = 0.5 * 0.75 = 0.375

The fact that Chile is a 50/50 makes all the math pretty straight forward. The typical question here is at least one, which would involve either calculating three probabilities or taking the compliment of visiting neither.

Hi Ron, Can you please tell me whats wrong if we find probability by doing: C OR M-(C&M)??

Hey Swarman, great question! A lot of people just plug in the formula and figure it will yield the right answer, but in this case you'd likely go directly to the trap answer C (without passing go or collecting 200$)

Let's look at the formula on a simple case. 2 fair coins, and we want to know if we'll get at least one Heads. We'd use the formula to say that the chance of getting heads on the first flip is 0.5, heads on the second is 0.5, and heads on both is 0.5x0.5 or 0.25. This gives us 0.5 + 0.5 - 0.25 = 0.75, which is correct. However, the question being asked here is "at least one head", which means HH is being counted in both calculations of 0.5. That's why we need to remove it afterwards, because it's being double counted, and should only be counted once.

In this example, the question is asking for exactly one or the other, which means both should be discarded. In the above coinflip example, there would be four possible outcomes: HH, HT, TH and TT. The question of at least one heads allows for HH, HT or TH. The question of exactly one heads allows for HT or TH, but not HH or TT. The question stem can dramatically change the question and send even the most experienced test taker towards a trap.

If you wanted to solve this by a formula, the same logic would apply, except it would be C or M - 2x(C&M). Logically, HH is being counted twice, when it should be counted zero times, so you must remove both instances of it from the equation. You can use this formula if you want, but the underlying logic is probably more helpful when faced with a tricky question on test day.

Re: There is a 50% chance Walcir will visit Chile this year [#permalink]

Show Tags

11 Mar 2013, 18:36

2

This post received KUDOS

3

This post was BOOKMARKED

emmak wrote:

There is a 50% chance Walcir will visit Chile this year, while there is a 25% chance that he will visit Madagascar this year. What is the probability that Walcir will visit either Chile or Madagascar this year, but NOT both?

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

Probability of visiting both Chile and Madagascar \(P=\frac{1}{2} X \frac{1}{4}\) \(P=\frac{1}{8}\)

Probability of not visiting Chile or Madagascar \(P=\frac{1}{2} X \frac{3}{4}\) \(P=\frac{3}{8}\)

In all other cases, Walcir will visit one or the other, but not both.

Probability of visiting Chile or Madagascar, but not both. \(P=1 - (\frac{1}{8} + \frac{3}{8})\) \(P=1 - \frac{4}{8}\) \(P=\frac{1}{2}\)

Re: There is a 50% chance Walcir will visit Chile this year [#permalink]

Show Tags

12 Mar 2013, 08:20

Thank you so much Ron I completely got what u r trying to say,but since you mentioned that understanding of the concept is more reqd can u pls tell me if what i have deduced is right ? ''This gives us 0.5 + 0.5 - 0.25 = 0.75''.. you found this as the probability of finding Atleast one head - which could be in HT/HH +TH/HH-HH and hence in the same question it has to be C or M - 2x(C&M) and NOT C or M -(C&M) as the latter would be STILL considering the option of both happening once.. right?

Thank you so much Ron I completely got what u r trying to say,but since you mentioned that understanding of the concept is more reqd can u pls tell me if what i have deduced is right ? ''This gives us 0.5 + 0.5 - 0.25 = 0.75''.. you found this as the probability of finding Atleast one head - which could be in HT/HH +TH/HH-HH and hence in the same question it has to be C or M - 2x(C&M) and NOT C or M -(C&M) as the latter would be STILL considering the option of both happening once.. right?

Hi swarman, exactly! C or M - (C*M) would mean you're still counting the possibility of both occurring once, which is why you have to remove it twice.

I mentioned that the understanding is more important than the formula(s) because the understanding will dictate how you use the formula(s), so you seem to be spot on here, which is great.

Try playing around with the question a little and seeing what it unlocks and what the test makers could try and lure you in with (example chances of going to Chile are 50% and Madagascar are 100%, or 0% and 50%, etc. You'll understand the problem from 360 degrees if you can feel comfortable with all the potential possibilities. That's what I try to do, but I'm a giant GMAT nerd

Re: There is a 50% chance Walcir will visit Chile this year [#permalink]

Show Tags

24 Mar 2013, 02:41

After reading all posts above, i got messed up with the basic concepts. "Let's look at the formula on a simple case. 2 fair coins, and we want to know if we'll get at least one Heads. We'd use the formula to say that the chance of getting heads on the first flip is 0.5, heads on the second is 0.5, and heads on both is 0.5x0.5 or 0.25. This gives us 0.5 + 0.5 - 0.25 = 0.75, which is correct. " If at least one head is needed in two flips, then propabilty should be " prob of head occuring in first coin (HT--->0.5) + prob of head occuring in second coin (HT--->0.5) + prob of head occuring in both the flips HH--->0.25 ( because we need that at least one head is occurring). So , all the three probabilties shud be added. Y do v subtract the last one...and how its like " the question being asked here is "at least one head", which means HH is being counted in both calculations of 0.5. That's why we need to remove it afterwards, because it's being double counted, and should only be counted once." ??? ....in ques it is mentioned that "AT LEAST" one head shud occur...so whats wrong in adding the third probabilty in which both heads are occurring.

After reading all posts above, i got messed up with the basic concepts. "Let's look at the formula on a simple case. 2 fair coins, and we want to know if we'll get at least one Heads. We'd use the formula to say that the chance of getting heads on the first flip is 0.5, heads on the second is 0.5, and heads on both is 0.5x0.5 or 0.25. This gives us 0.5 + 0.5 - 0.25 = 0.75, which is correct. " If at least one head is needed in two flips, then propabilty should be " prob of head occuring in first coin (HT--->0.5) + prob of head occuring in second coin (HT--->0.5) + prob of head occuring in both the flips HH--->0.25 ( because we need that at least one head is occurring). So , all the three probabilties shud be added. Y do v subtract the last one...and how its like " the question being asked here is "at least one head", which means HH is being counted in both calculations of 0.5. That's why we need to remove it afterwards, because it's being double counted, and should only be counted once." ??? ....in ques it is mentioned that "AT LEAST" one head shud occur...so whats wrong in adding the third probabilty in which both heads are occurring.

Hi Perhaps,

I apologize if the example confused you, I'm trying to denote the difference between the standard question: at least one occurs and this particular question: Exactly one occurs

AT LEAST ONE

If at least one occurs, you use the formula P(1) + P(2) - P(1&2). This is because the probability of 1&2 occuring is accounted for once in P(1) and another time in P(2). Hence, it is double counted. You then have to remove one instance of both occuring in order to get the correct answer.

In the simple example of two fair coin flips. The chances of getting Heads on at least one is P(1) + P(2) - P(1&2) or 0.5 + 0.5 - 0.25, which is 0.75. Written out: HH, HT and TH work. TT does not.

EXACTLY ONE

In this question, they are asking the equivalent of getting exactly one Head. Using the same basic formula, we could just take the probability of at least one and subtract the probability of both occuring, which leaves us with P(1) + P(2) - 2xP(1&2). As previously mentioned, the case where both occurs is accounted for twice when it should be accounted for zero times.

For the coin flips we again get P(1) + P(2) - 2xP(1&2) = 0.5 + 0.5 - 2x0.25 = 0.5. Written out: HT and TH work. HH and TT don't work.

On simple examples like this you're probably better off using logic and writing out examples (until ~10 possibilities this is best), but if the exam started asking about 5 coin flips you're better served to use the formulae than writing out 32 possibilities. I hope this helped clear up any misconceptions. The same principle can be applied regardless of the percentages used, it will only change when the number of coinflips or requirements change.

Re: There is a 50% chance Walcir will visit Chile this year [#permalink]

Show Tags

10 Sep 2014, 15:11

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________

There is a 50% chance Walcir will visit Chile this year, while there is a 25% chance that he will visit Madagascar this year. What is the probability that Walcir will visit either Chile or Madagascar this year, but NOT both?

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

Responding to a pm:

Quote:

what is the best way to solve these can we use venn diagrams to solve this

The best way is the one that comes to your mind as long as it leads you to the answer in the stipulated time!

As for using venn diagrams here, you don't really need to as long as you understand the logic of venn diagrams (sets). There are two events - visiting Chile (1/2) and visiting Madagascar (1/4). The probability that both may take place = (1/2)*(1/4) = 1/8 (independent events)

P(Chile or Madagascar or both) = 1/2 + 1/4 - 1/8 (from sets, we know this. We remove the extra common region once)

Now what do you do if you want to remove "both" from this? You subtract another 1/8 to totally remove the common region. You get P(Chile only or Madagascar only) = 1/2 + 1/4 -1/8 - 1/8 = 1/2

Or another way to think about is this: P(Visiting Chile) = 1/2. From this remove the probability of both. P(Visiting Madagascar) = 1/4. From this remove the probability of both.

P(Visiting Chile only or Visiting Madagascar only) = 1/2 - 1/8 + 1/4 - 1/8 = 1/2 _________________

Re: There is a 50% chance Walcir will visit Chile this year [#permalink]

Show Tags

11 Oct 2015, 20:58

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________

Check out this awesome article about Anderson on Poets Quants, http://poetsandquants.com/2015/01/02/uclas-anderson-school-morphs-into-a-friendly-tech-hub/ . Anderson is a great place! Sorry for the lack of updates recently. I...

As you leave central, bustling Tokyo and head Southwest the scenery gradually changes from urban to farmland. You go through a tunnel and on the other side all semblance...

Ghibli studio’s Princess Mononoke was my first exposure to Japan. I saw it at a sleepover with a neighborhood friend after playing some video games and I was...