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:

A medical researcher must choose one of 14 patients to recei [#permalink]

Show Tags

17 Dec 2011, 05:20

00:00

A

B

C

D

E

Difficulty:

(N/A)

Question Stats:

100% (01:52) correct
0% (00:00) wrong based on 0 sessions

HideShow timer Statistics

A medical researcher must choose one of 14 patients to receive an experimental medicine called Progaine. The researcher must then choose one of the remaining 13 patients to receive another medicine, called Ropecia. Finally, the researcher administers a placebo to one of the remaining 12 patients. All choices are equally random. If Donald is one of the 14 patients, what is the probability that Donald receives either Progaine or Ropecia?

A medical researcher must choose one of 14 patients to receive an experimental medicine called Progaine. The researcher must then choose one of the remaining 13 patients to receive another medicine, called Ropecia. Finally, the researcher administers a placebo to one of the remaining 12 patients. All choices are equally random. If Donald is one of the 14 patients, what is the probability that Donald receives either Progaine or Ropecia?

OA = 1/7

Here is how I approached this question -

Probability that Don gets P = 1/14 Probability that Don gets R = (prob he doesn't get P)(prob he gets R) = 13/14*1/13 = 1/14

Prob he gets P or R = Prob he gets P + Prob he gets R = 1/14 + 1/14 = 1/7

A medical researcher must choose one of 14 patients to receive an experimental medicine called Progaine. The researcher must then choose one of the remaining 13 patients to receive another medicine, called Ropecia. Finally, the researcher administers a placebo to one of the remaining 12 patients. All choices are equally random. If Donald is one of the 14 patients, what is the probability that Donald receives either Progaine or Ropecia?

OA = 1/7

My answer is different - Donald receives Progaine = (1)*(1/13)*(1/12) Donald receives Ropecia = (1/14)*(1)*(1/12)

Total - (1)*(1/13)*(1/12)+(1/14)*(1)*(1/12) = 9/728 Cheers! _________________

----------------------------------------------------------------------------------------- What you do TODAY is important because you're exchanging a day of your life for it! -----------------------------------------------------------------------------------------

This is an " or" and an "and" operatiob. In prpbabilty, or is equivalent to adding and "and" to multiplying. So the answer is the probability of getting the first drug (1/14) OR (+) the probability of getting the second drug which is equivalent to dont get the first drug (13/14) AND (*) getting the second drug (1/13). So the operation is: (1/14) + (13/14)*(1/13) = 1/14 + 1/14 = 1/7

We only need to find the probability that Donald receives either Progaine or Ropecia. By multiplying by 1/12 you are trying to take into account the placebo round. However, whether Donald is around to be chosen for the placebo does not affect the probability of him getting a dosage of either Progaine or Ropecia.

Had the question been asking for the probability of Donald receiving a placebo the answer would be (13/14)(12/13)(1/12) = 1/14.

We only need to find the probability that Donald receives either Progaine or Ropecia. By multiplying by 1/12 you are trying to take into account the placebo round. However, whether Donald is around to be chosen for the placebo does not affect the probability of him getting a dosage of either Progaine or Ropecia.

Had the question been asking for the probability of Donald receiving a placebo the answer would be (13/14)(12/13)(1/12) = 1/14.

Hope that helps!

@ Chris - got it, thanks. So the operation should be : (1/14) + (13/14)*(1/13) = 1/14 + 1/14 = 1/7

What you say? _________________

----------------------------------------------------------------------------------------- What you do TODAY is important because you're exchanging a day of your life for it! -----------------------------------------------------------------------------------------

There's no need for any multiplications here. The chance anyone gets Progaine is 1/14, so the chance Don gets Progaine is 1/14. The chance anyone gets Ropecia is 1/14, so the chance Don gets Ropecia is 1/14. So the chance he gets one of the two is 1/14 + 1/14 = 1/7.

Or you can just imagine lining the people up at random, and giving the first two people in line Progaine and Ropecia. The chance Don is among the first two people is 2/14 = 1/7. _________________

GMAT Tutor in Toronto

If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com

There's no need for any multiplications here. The chance anyone gets Progaine is 1/14, so the chance Don gets Progaine is 1/14. The chance anyone gets Ropecia is 1/14, so the chance Don gets Ropecia is 1/14. So the chance he gets one of the two is 1/14 + 1/14 = 1/7.

Or you can just imagine lining the people up at random, and giving the first two people in line Progaine and Ropecia. The chance Don is among the first two people is 2/14 = 1/7.

My approach was same as the popular one here. I got the right answer, but the official explanation was as given by IStewart. I still can't get this explanation of 1/14+1/14. Still, the other way i.e. giving the first two people in line Progaine and Ropecia. The chance Don is among the first two people is 2/14 = 1/7 makes some sense. _________________

Re: A medical researcher must choose one of 14 patients to recei [#permalink]

Show Tags

18 Feb 2014, 11:41

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. _________________

Re: A medical researcher must choose one of 14 patients to recei [#permalink]

Show Tags

18 Feb 2014, 13:11

well here is an alternate solution for those people who still wants to follow the multiplication method.

For the sake of simplicity lets name three medicines as A,B and C.

Now out of 14 people, anyone can be selected for the medicine A in 14C1 ways out of remaining 13 people, anyone can be selected for medicine B in 13C1 ways Lastly from remaining 12 people, anyone can be selected for medicine C in 12C1 ways

hence total no. of ways for selecting 3 person for medicine A,B and C is 14C1x13C1x12C1 = 14x13x12

Now donald can get either medicine A or medicine B Case 1 if he gets medicine A, then person for medicine B can be selected in 13C1 ways and person for medicine C can be selected in 12C1 ways hence total no. of ways = 1x13C1x12C1

Case 2 if he gets medicine B, then person for medicine A can be selected in 13C1 ways and person for medicine C can be selected in 12C1 ways hence total no. of ways= 1x13C1x12C1

hence total no. of favorable ways= case1 +case2 = 2x13x12

hence required probability = (2x13x12)/(14x13x12) =1/7

Re: A medical researcher must choose one of 14 patients to recei [#permalink]

Show Tags

19 Feb 2014, 00:22

Expert's post

A medical researcher must choose one of 14 patients to receive an experimental medicine called Progaine. The researcher must then choose one of the remaining 13 patients to receive another medicine, called Ropecia. Finally, the researcher administers a placebo to one of the remaining 12 patients. All choices are equally random. If Donald is one of the 14 patients, what is the probability that Donald receives either Progaine or Ropecia?

Donald to receiver either Prograine or Ropecia must be among first two chosen patients and as there are 14 patients then the probability of this is simply 2/14=1/7.

Excellent posts dLo saw your blog too..!! Man .. you have got some writing skills. And Just to make an argument = You had such an amazing resume ; i am glad...

So Much $$$ Business school costs a lot. This is obvious, whether you are a full-ride scholarship student or are paying fully out-of-pocket. Aside from the (constantly rising)...

They say you get better at doing something by doing it. then doing it again ... and again ... and again, and you keep doing it until one day you look...