GMAT Question of the Day - Daily to your Mailbox; hard ones only

It is currently 15 Nov 2018, 04:38

ISB R1 results on Nov 15:

Join Chat Room for Live Updates


Close

GMAT Club Daily Prep

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.

Close

Request Expert Reply

Confirm Cancel
Events & Promotions in November
PrevNext
SuMoTuWeThFrSa
28293031123
45678910
11121314151617
18192021222324
2526272829301
Open Detailed Calendar
  • Free GMAT Strategy Webinar

     November 17, 2018

     November 17, 2018

     07:00 AM PST

     09:00 AM PST

    Nov. 17, 7 AM PST. Aiming to score 760+? Attend this FREE session to learn how to Define your GMAT Strategy, Create your Study Plan and Master the Core Skills to excel on the GMAT.
  • GMATbuster's Weekly GMAT Quant Quiz # 9

     November 17, 2018

     November 17, 2018

     09:00 AM PST

     11:00 AM PST

    Join the Quiz Saturday November 17th, 9 AM PST. The Quiz will last approximately 2 hours. Make sure you are on time or you will be at a disadvantage.

7^3 + 21^3 =

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:

Hide Tags

Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 50608
7^3 + 21^3 =  [#permalink]

Show Tags

New post 13 Feb 2017, 02:16
3
6
00:00
A
B
C
D
E

Difficulty:

  25% (medium)

Question Stats:

71% (00:58) correct 29% (01:26) wrong based on 175 sessions

HideShow timer Statistics

Intern
Intern
avatar
B
Joined: 08 Sep 2016
Posts: 35
Location: Georgia
Concentration: Finance, International Business
GPA: 3.75
WE: Analyst (Investment Banking)
7^3 + 21^3 =  [#permalink]

Show Tags

New post 13 Feb 2017, 02:56
2
7^3+21^3
7^3 + 7^3*3^3 => 7^3(1+3^3)= 7^3*28=7^3*7*4= 7^4*2^2

answer is a
VP
VP
User avatar
P
Joined: 05 Mar 2015
Posts: 1000
Re: 7^3 + 21^3 =  [#permalink]

Show Tags

New post 13 Feb 2017, 05:20
1
Bunuel wrote:
\(7^3 + 21^3 =\)

A. \(7^4*2^2\)

B. \(7^3*2^4\)

C. \(7^3*3^3\)

D. \(7^3*3^4\)

E. \(7^4*3^3\)



21^3 = (7*3)^3 = 7^3*3^3

7^3 + 21^3 taking 7^3 common
7^3(1+3^3) = 7^3(28)
28= 7*4 = 7*2^2

thus 7^3(7*2^2)
= 7^4*2^2

Ans A
Board of Directors
User avatar
P
Status: QA & VA Forum Moderator
Joined: 11 Jun 2011
Posts: 4209
Location: India
GPA: 3.5
WE: Business Development (Commercial Banking)
GMAT ToolKit User Premium Member
Re: 7^3 + 21^3 =  [#permalink]

Show Tags

New post 13 Feb 2017, 08:26
4
Bunuel wrote:
\(7^3 + 21^3 =\)

A. \(7^4*2^2\)

B. \(7^3*2^4\)

C. \(7^3*3^3\)

D. \(7^3*3^4\)

E. \(7^4*3^3\)


\(7^3 + 21^3\)

= \(7^3 + 7^3*3^3\)

= \(7^3 (1 +3^3 )\)

= \(7^3 *28\)

= \(7^3 *7*2^2\)

= \(7^4 *2^2\)

Hence, answer will be (A) \(7^4 *2^2\)
_________________

Thanks and Regards

Abhishek....

PLEASE FOLLOW THE RULES FOR POSTING IN QA AND VA FORUM AND USE SEARCH FUNCTION BEFORE POSTING NEW QUESTIONS

How to use Search Function in GMAT Club | Rules for Posting in QA forum | Writing Mathematical Formulas |Rules for Posting in VA forum | Request Expert's Reply ( VA Forum Only )

Target Test Prep Representative
User avatar
P
Status: Founder & CEO
Affiliations: Target Test Prep
Joined: 14 Oct 2015
Posts: 4170
Location: United States (CA)
Re: 7^3 + 21^3 =  [#permalink]

Show Tags

New post 14 Feb 2017, 15:50
1
Bunuel wrote:
\(7^3 + 21^3 =\)

A. \(7^4*2^2\)

B. \(7^3*2^4\)

C. \(7^3*3^3\)

D. \(7^3*3^4\)

E. \(7^4*3^3\)


We can simplify the given equation:

7^3 + 21^3

7^3 + (7x3)^3

7^3 + 7^3 x 3^3

7^3(1 + 3^3)

7^3(1 + 27)

7^3(28)

7^3 x 7^1 x 2^2

7^4 x 2^2

Answer: A
_________________

Scott Woodbury-Stewart
Founder and CEO

GMAT Quant Self-Study Course
500+ lessons 3000+ practice problems 800+ HD solutions

CEO
CEO
User avatar
D
Joined: 11 Sep 2015
Posts: 3116
Location: Canada
Re: 7^3 + 21^3 =  [#permalink]

Show Tags

New post 15 Feb 2017, 08:06
1
Top Contributor
2
Bunuel wrote:
\(7^3 + 21^3 =\)

A. \(7^4 * 2^2\)

B. \(7^3*2^4\)

C. \(7^3*3^3\)

D. \(7^3*3^4\)

E. \(7^4*3^3\)


This question can also be solved without using any exponent laws.

First, we'll use the fact that the product of a bunch of odd integers will always be odd.
And we'll use the fact that, if a product contains 1 or more even integers, then that product will be even

So, 7³ + 21³ = (some odd number)³ + (some odd number)³
= ODD + ODD
= EVEN
So, 7³ + 21³ = an even number

Now check the answer choices....
A. \(7^4 * 2^2\) = \(ODD^4*EVEN^2\) = ODD * EVEN = EVEN (Keep)

B. \(7^3*2^4\) = \(ODD^3*EVEN^4\) = ODD * EVEN = EVEN (Keep)

C. \(7^3*3^3\) = \(ODD^3 * ODD^3\) = ODD * ODD = ODD (eliminate)

D. \(7^3*3^4\) = \(ODD^3 * ODD^4\) = ODD * ODD = ODD (eliminate)

E. \(7^4*3^3\) = \(ODD^4 * ODD^3\) = ODD * ODD = ODD (eliminate)

So, the correct answer is A or B

At this point, let's focus on the UNITS DIGITS ONLY

\(7^3\) = (7)(7)(7) = ????3 [some number with units digit 3]
\(21^3 =\) (21)(21)(21) = ????1 [some number with units digit 1]
So, \(7^3 + 21^3 =\) ????3 + ????1 = ????4
Now we'll check the remaining answer choices to see which one yields an answer with units digit 4

A. \(7^4 * 2^2\)
\(7^4\) = (7)(7)(7)(7) = ????1 [some number with units digit 1]
\(2^2\) = (2)(2) = 4
So, \(7^4 * 2^2\) = (????1)(4) = ??????4
Great - keep A

B. \(7^3*2^4\)
\(7^3\) = (7)(7)(7) = ????3
\(2^2\) = (2)(2)(2)(2) = 16
So, \(7^3 * 2^4\) = (????3)(16) = ??????8
ELIMINATE B

Answer: A
_________________

Brent Hanneson – GMATPrepNow.com
Image

Non-Human User
User avatar
Joined: 09 Sep 2013
Posts: 8774
Premium Member
Re: 7^3 + 21^3 =  [#permalink]

Show Tags

New post 22 Oct 2018, 05:29
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.
_________________

GMAT Books | GMAT Club Tests | Best Prices on GMAT Courses | GMAT Mobile App | Math Resources | Verbal Resources

GMAT Club Bot
Re: 7^3 + 21^3 = &nbs [#permalink] 22 Oct 2018, 05:29
Display posts from previous: Sort by

7^3 + 21^3 =

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  


Copyright

GMAT Club MBA Forum Home| About| Terms and Conditions and Privacy Policy| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.