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

Redeem

Operations with Rational Numbers

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Newbie | Next Rank: 10 Posts
Posts: 6
Joined: Thu Sep 29, 2016 10:34 am

Operations with Rational Numbers

by jmacym » Sat Oct 08, 2016 10:42 am

Timer

00:00

Your Answer

A

B

C

D

E

Global Stats

One of my review questions...

The sum 7/8 + 1/9 is between...

The correct answer is (B) 3/4 and 1

The explanation is as follows: Since 1/9 < 1/8, 7/8 + 1/9 < 7/8 + 1/8 = 1 answer choices C, D, and E can be eliminated.
Since 7/8 > 6/8 = 3/4, 7/8 + 1/9 > 3/4 answer choice A can be eliminated. Thus, 3/4 < 7/8 + 1/9 < 1

Okay, so my thinking is first of all 7/8 + 1/9 is 71/72... obviously that was not even close to any of the answer choices.
The explanation confuses me even more because it's saying 1/9 is less than 1/8...which I understand how and why that is true, but 1/9 could be less than TONS of numbers. How am I supposed to know to use 1/8 instead of, lets say, 1/7 or 1/6, 1/5..etc. to get to the correct answer?
Top
Source: — Problem Solving |
User avatar
Legendary Member
Posts: 2663
Joined: Wed Jan 14, 2015 8:25 am
Location: Boston, MA
Thanked: 1153 times
Followed by:128 members
GMAT Score:770

by DavidG@VeritasPrep » Sat Oct 08, 2016 10:49 am
jmacym wrote:One of my review questions...

The sum 7/8 + 1/9 is between...

The correct answer is (B) 3/4 and 1

The explanation is as follows: Since 1/9 < 1/8, 7/8 + 1/9 < 7/8 + 1/8 = 1 answer choices C, D, and E can be eliminated.
Since 7/8 > 6/8 = 3/4, 7/8 + 1/9 > 3/4 answer choice A can be eliminated. Thus, 3/4 < 7/8 + 1/9 < 1

Okay, so my thinking is first of all 7/8 + 1/9 is 71/72... obviously that was not even close to any of the answer choices.
The explanation confuses me even more because it's saying 1/9 is less than 1/8...which I understand how and why that is true, but 1/9 could be less than TONS of numbers. How am I supposed to know to use 1/8 instead of, lets say, 1/7 or 1/6, 1/5..etc. to get to the correct answer?
The idea is that (7/8) + (1/8) = 1. Because 1/9 < 1/8, it must be true that (7/8) + (1/9) < 1.

(In essence, 1/8 was chosen because it allowed you to easily see how the sum would compare to one.)
Veritas Prep | GMAT Instructor

Veritas Prep Reviews
Save $100 off any live Veritas Prep GMAT Course
Top

GMAT/MBA Expert

User avatar
Elite Legendary Member
Posts: 10392
Joined: Sun Jun 23, 2013 6:38 pm
Location: Palo Alto, CA
Thanked: 2867 times
Followed by:511 members
GMAT Score:800

by [email protected] » Sat Oct 08, 2016 6:58 pm
Hi jmacym,

When posting a GMAT question, you should make sure to post the ENTIRE question (including the 5 answer choices). In many cases, the answer choices themselves provide a 'hint' as to how you can go about answering the question. The question that you're referring to includes the following answers:

The sum 7/8 + 1/9 is between?

A) 1/2 and 3/4
B) 3/4 and 1
C) 1 and 1 1/4
D) 1 1/4 and 1 1/2
E) 1 1/2 and 2

From the answer choices, we can see that there is no 'overlap' between any pair of consecutive answers, so we really just need to define the range. By extension, we likely do NOT need to do a perfect calculation to get to the correct answer (we can probably use a certain amount of estimation).

Since 6/8 = 3/4 = .75, we know that 7/8 has to be greater than .75 on its own.

7/8 + 1/8 would = 1, but since 1/9 < 1/8, then we're adding a number to .75 that will end up in a sum that is less than 1. We've now defined the range...

Final Answer: B

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
Image
Top
User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pm
Location: New York, NY
Thanked: 13060 times
Followed by:1906 members
GMAT Score:790

by GMATGuruNY » Sat Oct 08, 2016 7:02 pm
jmacym wrote:The sum 7/8 + 1/9 is between?

A) 1/2 and 3/4
B) 3/4 and 1
C) 1 and 1 1/4
D) 1 1/4 and 1 1/2
E) 1 1/2 and 2
7/8 + 1/8 = 1.
Since 1/9 is just a bit SMALLER than 1/8, 7/8 + 1/9 must be just a BIT LESS THAN 1.

The correct answer is B.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 16207
Joined: Mon Dec 08, 2008 6:26 pm
Location: Vancouver, BC
Thanked: 5254 times
Followed by:1268 members
GMAT Score:770

by Brent@GMATPrepNow » Sun Oct 09, 2016 7:25 am
The sum 7/8 + 1/9 is between

A)1/2 and 3/4
B)3/4 and 1
C)1 and 1 1/4
D)1 1/4 and 1 1/2
E)1 1/2 and 2
Well, first we know that 7/8 + 1/8 = 1
Since 1/9 is a little bit less than 1/8, we know that the sum 7/8 + 1/9 will be a little bit less than 1.
Since 7/8 is also greater than 3/4, the answer must be B
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
User avatar
GMAT Instructor
Posts: 1462
Joined: Thu Apr 09, 2015 9:34 am
Location: New York, NY
Thanked: 39 times
Followed by:22 members

by Jeff@TargetTestPrep » Fri Apr 06, 2018 7:59 am
The sum 7/8 + 1/9 is between

A)1/2 and 3/4
B)3/4 and 1
C)1 and 1 1/4
D)1 1/4 and 1 1/2
E)1 1/2 and 2
Notice that 1/9 is less than 1/8, so 7/8 + 1/9 < 7/8 + 1/8 = 1.

Furthermore, 7/8 itself is greater than 3/4 since 3/4 = 6/8.

Thus we see that the sum is between 3/4 and 1.

Alternate Solution:

Let's add the two fractions, using 72 as the common denominator:

7/8 + 1/9 = 63/72 + 8/72 = 71/72.

We see that 71/72 is just slightly less than 1, so B is the obvious choice..

Answer: B

Jeffrey Miller
Head of GMAT Instruction
[email protected]

Image

See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews
Top
Post new topic Post Reply
• Page 1 of 1