• Economist Test Prep
    Free Trial & Practice Exam
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    Economist Test Prep
  • EMPOWERgmat Slider
    1 Hour Free
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    EMPOWERgmat Slider
  • Kaplan Test Prep
    Free Practice Test & Review
    How would you score if you took the GMAT

    Available with Beat the GMAT members only code

    MORE DETAILS
    Kaplan Test Prep
  • e-gmat Exclusive Offer
    Get 300+ Practice Questions
    25 Video lessons and 6 Webinars for FREE

    Available with Beat the GMAT members only code

    MORE DETAILS
    e-gmat Exclusive Offer
  • Varsity Tutors
    Award-winning private GMAT tutoring
    Register now and save up to $200

    Available with Beat the GMAT members only code

    MORE DETAILS
    Varsity Tutors
  • Veritas Prep
    Free Veritas GMAT Class
    Experience Lesson 1 Live Free

    Available with Beat the GMAT members only code

    MORE DETAILS
    Veritas Prep
  • Magoosh
    Magoosh
    Study with Magoosh GMAT prep

    Available with Beat the GMAT members only code

    MORE DETAILS
    Magoosh
  • PrepScholar GMAT
    5 Day FREE Trial
    Study Smarter, Not Harder

    Available with Beat the GMAT members only code

    MORE DETAILS
    PrepScholar GMAT
  • Target Test Prep
    5-Day Free Trial
    5-day free, full-access trial TTP Quant

    Available with Beat the GMAT members only code

    MORE DETAILS
    Target Test Prep

Probability Question

This topic has 4 expert replies and 3 member replies
chiphipoke Newbie | Next Rank: 10 Posts Default Avatar
Joined
03 Dec 2013
Posted:
1 messages

Probability Question

Post Mon Jan 13, 2014 8:22 am
Of the 20 members of a kitchen crew, 17 can use the meat-cutting machine, 18 can use the bread-slicing machine, and 15 can use both machines. If one member of the crew is to be chose at random, what is the probability that the member chosen will be someone who cannot use either machine?


    0
    1/10
    1/7
    1/4
    1/3


This question is from GMATPrep's Exam Pack 3 and I cannot solve it for the life of me. Please provide an explanation. Thanks!

Answer is A

  • +1 Upvote Post
  • Quote
  • Flag
Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
petrorich Newbie | Next Rank: 10 Posts Default Avatar
Joined
11 Jan 2013
Posted:
3 messages
Post Tue Jun 23, 2015 8:37 am
I was racking my brain on this problem, until I figured out you can use a Venn Diagram to make life easier!!

Had I not been so narrow-minded and always trying to solve problems using ONLY 1 method, I would have gotten this within seconds!


ttp://postimg.org/image/7cd3onccf/" target="_blank">

  • +1 Upvote Post
  • Quote
  • Flag

GMAT/MBA Expert

Matt@VeritasPrep GMAT Instructor
Joined
12 Sep 2012
Posted:
2640 messages
Followed by:
113 members
Upvotes:
625
Target GMAT Score:
V51
GMAT Score:
780
Post Tue Jun 23, 2015 6:01 pm
Petro, that's a great idea: any time you have two groups with some overlap, a Venn (or a matrix) is a lifesaver.

  • +1 Upvote Post
  • Quote
  • Flag
Enroll in a Veritas Prep GMAT class completely for FREE. Wondering if a GMAT course is right for you? Attend the first class session of an actual GMAT course, either in-person or live online, and see for yourself why so many students choose to work with Veritas Prep. Find a class now!

GMAT/MBA Expert

Matt@VeritasPrep GMAT Instructor
Joined
12 Sep 2012
Posted:
2640 messages
Followed by:
113 members
Upvotes:
625
Target GMAT Score:
V51
GMAT Score:
780
Post Wed Jan 15, 2014 12:15 pm
One last tip: even if you're completely lost on test day, you KNOW the answer can't be (C) or (E) because you can't have (1/7) or (1/3) of a group of 20 people - no fractional persons allowed! You know that the answer can't be (1/4): since you have 18 people out of 20 who can use the bread slicer, you have AT MOST 2 people (or 1/10 of the total) who can't use either machine.

With that, you know the answer is either A or B, and you have a 50% chance of getting the question right! Pretty cool, and almost more satisfying than actually knowing how to solve. Very Happy

chiphipoke wrote:
Of the 20 members of a kitchen crew, 17 can use the meat-cutting machine, 18 can use the bread-slicing machine, and 15 can use both machines. If one member of the crew is to be chose at random, what is the probability that the member chosen will be someone who cannot use either machine?


    0
    1/10
    1/7
    1/4
    1/3


This question is from GMATPrep's Exam Pack 3 and I cannot solve it for the life of me. Please provide an explanation. Thanks!

Answer is A

  • +1 Upvote Post
  • Quote
  • Flag
Enroll in a Veritas Prep GMAT class completely for FREE. Wondering if a GMAT course is right for you? Attend the first class session of an actual GMAT course, either in-person or live online, and see for yourself why so many students choose to work with Veritas Prep. Find a class now!

GMAT/MBA Expert

Post Mon Jan 13, 2014 12:38 pm
theCodeToGMAT wrote:
Use a Matrix Method..
Black ones are from question
Green ones are implied
Red Box is what we need to find.

{A}
A note to those who are unfamiliar with the "(Double) Matrix Method": This technique can be used for most questions featuring a population in which each member has two characteristics associated with it.
Here, we have a population of kitchen crew members, and the two characteristics are:
- CAN use the meat-cutting machine and CANNOT use the meat-cutting machine
- CAN use the bread-slicing machine and CANNOT use the bread-slicing machine

To learn more about this technique, watch our free video: http://www.gmatprepnow.com/module/gmat-word-problems?id=919

Then try these additional practice questions that can be solved using the Double Matrix Method:
- http://www.beatthegmat.com/mba/2011/05/05/random-double-matrix-question-1
- http://www.beatthegmat.com/mba/2011/05/09/random-double-matrix-question-2
- http://www.beatthegmat.com/mba/2011/05/16/random-double-matrix-question-3
- http://www.beatthegmat.com/ds-quest-t187706.html
- http://www.beatthegmat.com/overlapping-sets-questions-t183320.html
- http://www.beatthegmat.com/finance-majors-non-finance-majors-overlapping-set-question-t167425.html
- http://www.beatthegmat.com/ds-french-japanese-t222297.html
- http://www.beatthegmat.com/sets-t269449.html#692540
- http://www.beatthegmat.com/in-costume-for-halloween-t269355.html#692116

Cheers,
Brent

_________________
Brent Hanneson – Founder of GMATPrepNow.com
Use our video course along with Beat The GMAT's free 60-Day Study Guide

Check out the online reviews of our course
Come see all of our free resources

  • +1 Upvote Post
  • Quote
  • Flag
Thanked by: chiphipoke
GMAT Prep Now's comprehensive video course can be used in conjunction with Beat The GMAT’s FREE 60-Day Study Guide and reach your target score in 2 months!

GMAT/MBA Expert

Post Mon Jan 13, 2014 12:08 pm
chiphipoke wrote:
Of the 20 members of a kitchen crew, 17 can use the meat-cutting machine, 18 can use the bread-slicing machine, and 15 can use both machines. If one member of the crew is to be chose at random, what is the probability that the member chosen will be someone who cannot use either machine?


    0
    1/10
    1/7
    1/4
    1/3

We can use the following formula for two overlapping groups:

Total = Group 1 + Group 2 - Both + Neither.

The big idea with overlapping groups is to SUBTRACT THE OVERLAP.
When we count everyone in Group 1 (meat-cutters) and everyone in Group 2 (bread-slicers), those in BOTH groups (members who can use BOTH types of machines) get counted twice.
So that we don't double-count the members in both groups, we SUBTRACT THE OVERLAP from the total.

In the problem at hand:
Total = 20.
Group 1 = meat-cutters = 17.
Group 2 = bread-slicers = 18.
Both = 15.
Let N = the number who can use neither type of machine.

Plugging these values into the equation above, we get:
20 = 17 + 18 - 15 + N
N = 0.

Since 0 members can use neither type of machine, P(selecting a member who can use neither type of machine) = 0.

The correct answer is A.

_________________
Mitch Hunt
GMAT Private Tutor
GMATGuruNY@gmail.com
If you find one of my posts helpful, please take a moment to click on the "UPVOTE" icon.
Available for tutoring in NYC and long-distance.
For more information, please email me at GMATGuruNY@gmail.com.

  • +1 Upvote Post
  • Quote
  • Flag
Thanked by: chiphipoke
Free GMAT Practice Test How can you improve your test score if you don't know your baseline score? Take a free online practice exam. Get started on achieving your dream score today! Sign up now.
coreyj808 Newbie | Next Rank: 10 Posts Default Avatar
Joined
13 Jan 2014
Posted:
2 messages
Upvotes:
1
Post Mon Jan 13, 2014 11:39 am
I agree, you need to take your time on this or it's very easy to make a mistake.

Think of it like a box where you are adding up numbers to get to 20.

So you know the people who can use both machines is 15 - this tells us that there is an overlap of 15 for both sides.

[ 15 ]

Then, we look at meat cutters (17), so the excess would be 2

[ 15 ]+[ 2 Meat cutters]

and then the difference between both and Bread makers which equals 3

[ 15 both ]+[ 2MC ]+[ 3BM ] = [ 20 total ]

Since the total is 20, there is no way that you could not pick someone that is not qualified in either. Hope this helps!

  • +1 Upvote Post
  • Quote
  • Flag
Thanked by: chiphipoke
theCodeToGMAT Legendary Member
Joined
14 Aug 2012
Posted:
1556 messages
Followed by:
34 members
Upvotes:
448
Target GMAT Score:
750
GMAT Score:
650
Post Mon Jan 13, 2014 9:37 am
Use a Matrix Method..
Black ones are from question
Green ones are implied
Red Box is what we need to find.

{A}
Attachments

This post contains an attachment. You must be logged in to download/view this file. Please login or register as a user.


_________________
R A H U L

  • +1 Upvote Post
  • Quote
  • Flag
Thanked by: chiphipoke

Best Conversation Starters

1 Roland2rule 174 topics
2 lheiannie07 110 topics
3 ardz24 64 topics
4 LUANDATO 56 topics
5 swerve 46 topics
See More Top Beat The GMAT Members...

Most Active Experts

1 image description Brent@GMATPrepNow

GMAT Prep Now Teacher

150 posts
2 image description Jeff@TargetTestPrep

Target Test Prep

145 posts
3 image description Rich.C@EMPOWERgma...

EMPOWERgmat

116 posts
4 image description GMATGuruNY

The Princeton Review Teacher

103 posts
5 image description Scott@TargetTestPrep

Target Test Prep

98 posts
See More Top Beat The GMAT Experts