BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

OG Quant Review Q 76

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Legendary Member
Posts: 631
Joined: Mon Feb 18, 2008 11:57 pm
Thanked: 29 times
Followed by:3 members

OG Quant Review Q 76

by netigen » Sat Apr 05, 2008 5:56 pm
Today Rose is twice as old as Sam and Sam is 3 years younger than Tina. If Rose, Sam, and Tina are all alive 4 years from today, which of the following must be true on that day.

I. Rose is twice as old as Sam
II. Sam is 3 years younger than Tina
III. Rose is older than Tina

(A) I only
(B) II only
(C) III only
(D) I and II
(E) II and III

Ans given in the book is B
but I think the answer should be E

The book creates this equation for III

2(T-3)>T
T>6 which book say's that need not be true

but if we look at the facts in the questions since 4 years from now Sam will atleast be 4 which makes T atleast 7 so this equation does hold actually.
Top
Source: — Problem Solving |
User avatar
GMAT Instructor
Posts: 3225
Joined: Tue Jan 08, 2008 2:40 pm
Location: Toronto
Thanked: 1710 times
Followed by:614 members
GMAT Score:800

Re: OG Quant Review Q 76

by Stuart@KaplanGMAT » Sat Apr 05, 2008 6:11 pm
netigen wrote:Today Rose is twice as old as Sam and Sam is 3 years younger than Tina. If Rose, Sam, and Tina are all alive 4 years from today, which of the following must be true on that day.

I. Rose is twice as old as Sam
II. Sam is 3 years younger than Tina
III. Rose is older than Tina

(A) I only
(B) II only
(C) III only
(D) I and II
(E) II and III

Ans given in the book is B
but I think the answer should be E

The book creates this equation for III

2(T-3)>T
T>6 which book say's that need not be true

but if we look at the facts in the questions since 4 years from now Sam will atleast be 4 which makes T atleast 7 so this equation does hold actually.
What if we pick todays ages as:

Rose 2
Sam 1
Tina 4

so, 4 years from now we have:

rose 6
Sam 5
Tina 8

So:

I Rose is twice as old as sam - not true
II Sam is 3 years younger than tina - true
III Rose is older than Tina - not true

We've eliminated (a), (c), (d) and (e): choose (b).
Image

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
Top
Master | Next Rank: 500 Posts
Posts: 328
Joined: Thu Aug 07, 2008 5:25 pm
Location: Philadelphia
Thanked: 4 times
GMAT Score:550

by Abdulla » Sun Nov 23, 2008 11:00 am
Nice shot, however If we picked different numbers see bellows what gonna happen..

Today Ages:

Rose 14
Sam 7
Tina 10

so, 4 years from now we have:

Rose 18
Sam 11
Tina 14

So:

I Rose is twice as old as sam - not true
II Sam is 3 years younger than tina - true
III Rose is older than Tina - TRUE

Can you suggest different approach?
Abdulla
Top
User avatar
GMAT Instructor
Posts: 3225
Joined: Tue Jan 08, 2008 2:40 pm
Location: Toronto
Thanked: 1710 times
Followed by:614 members
GMAT Score:800

by Stuart@KaplanGMAT » Sun Nov 23, 2008 11:42 am
Abdulla wrote:Nice shot, however If we picked different numbers see bellows what gonna happen..

Today Ages:

Rose 14
Sam 7
Tina 10

so, 4 years from now we have:

Rose 18
Sam 11
Tina 14

So:

I Rose is twice as old as sam - not true
II Sam is 3 years younger than tina - true
III Rose is older than Tina - TRUE

Can you suggest different approach?
The question asks "which of the following MUST be true?", not "which of the following COULD be true?"

For this type of question, our goal is to pick numbers that show that a statement COULD be false, since if we do so we can eliminate it from contention.

By picking the numbers that I did, I showed that I and III need not be true (i.e. they could be false), allowing us to eliminate 4/5 answer choices. Although I didn't prove that statement II must be true, there's only one answer choice left, so we're 100% certain that it's correct.

By picking the numbers that you did, you should that I need not be true, allowing you to eliminate (A) and (D). However, you haven't proven that II and III MUST be true, so choices (B), (C) and (E) are all still potential answers to the question. Now you need to see if you can pick another set of numbers that shows that either II or III could be false.

The disadvantage of picking numbers is that, unless you narrow it down to 1 choice, you may not be 100% sure that you've covered every possibility. Accordingly, you need to pick different "kinds" of numbers (e.g. as big as possible, as small as possible) to build your confidence that you've exhausted the options.
Image

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
Top
Master | Next Rank: 500 Posts
Posts: 328
Joined: Thu Aug 07, 2008 5:25 pm
Location: Philadelphia
Thanked: 4 times
GMAT Score:550

by Abdulla » Sun Nov 23, 2008 11:57 am
That means when I picked numbers I have to pick two different sets to make sure that I'm not missing any possibility ?
Abdulla
Top
User avatar
GMAT Instructor
Posts: 3225
Joined: Tue Jan 08, 2008 2:40 pm
Location: Toronto
Thanked: 1710 times
Followed by:614 members
GMAT Score:800

by Stuart@KaplanGMAT » Sun Nov 23, 2008 1:02 pm
Abdulla wrote:That means when I picked numbers I have to pick two different sets to make sure that I'm not missing any possibility ?
Well, if you pick the right numbers the first time, you'll only have to pick one set. However, there are times when you'll want to pick a second (or even third) set to make sure that you've covered all the possibilities.

The types of number you choose should depend on the question. For example, in an odd/even question, once you've chosen both odds and evens, you don't need to try anything else. In a prime number question, once you've chosen 2 and just about any other prime, you're usually done. In a positive/negative question, you usually want to try a positive, a negative and zero.

Understanding which types of numbers to pick in which situations will save you a lot of time on test day, which of course will lead to more points.
Image

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
Top
Master | Next Rank: 500 Posts
Posts: 328
Joined: Thu Aug 07, 2008 5:25 pm
Location: Philadelphia
Thanked: 4 times
GMAT Score:550

by Abdulla » Sun Nov 23, 2008 6:11 pm
Thanks Stuart .. got you
Abdulla
Top
Post new topic Post Reply
• Page 1 of 1