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

It is currently 17 Jul 2018, 16:28

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

If a, b, x and y are prime numbers and a^x + b^y = 129, what is the va

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

Hide Tags

2 KUDOS received
Intern
Intern
avatar
Joined: 23 Mar 2014
Posts: 12
GMAT ToolKit User
If a, b, x and y are prime numbers and a^x + b^y = 129, what is the va [#permalink]

Show Tags

New post 14 Sep 2014, 15:33
2
4
00:00
A
B
C
D
E

Difficulty:

  25% (medium)

Question Stats:

73% (01:40) correct 27% (01:51) wrong based on 179 sessions

HideShow timer Statistics

If a, b, x and y are prime numbers and a^x + b^y = 129, what is the value of x+y?

(A) 5
(B) 6
(C) 10
(D) 13
(E) 15

Can someone explain how to approach this question systematically? Ie. what are your first couple steps in handling this question and how do you break it down?
Most Helpful Community Reply
4 KUDOS received
Manager
Manager
User avatar
S
Joined: 22 Jan 2014
Posts: 167
WE: Project Management (Computer Hardware)
Re: If a, b, x and y are prime numbers and a^x + b^y = 129, what is the va [#permalink]

Show Tags

New post 26 Sep 2014, 22:59
4
1
beef001 wrote:
If a, b, x and y are prime numbers and a^x + b^y = 129, what is the value of x+y?

(A) 5
(B) 6
(C) 10
(D) 13
(E) 15

Can someone explain how to approach this question systematically? Ie. what are your first couple steps in handling this question and how do you break it down?


a^x + b^y = 129 (odd)
all primes except 2 are odd. but if a and b are odd primes then odd+odd = even
so one of a or b has to be 2.
2^x + b^y = 129
2^2 + 5^3 = 129 ;)
_________________

Illegitimi non carborundum.

General Discussion
Intern
Intern
avatar
Joined: 08 Jul 2014
Posts: 1
Re: If a, b, x and y are prime numbers and a^x + b^y = 129, what is the va [#permalink]

Show Tags

New post 14 Sep 2014, 17:10
Hi, It should not be difficult, since simple testing 2 pares of primes gives the result.
a(x) + b(y) = 129
11(2) + 2(3) = 129
x+y = 5

I think, that such questions do not have system approach, as they generally test the skills of correct Picking of numbers...
Manager
Manager
avatar
Joined: 10 Feb 2014
Posts: 113
GMAT 1: 690 Q50 V33
GMAT ToolKit User
Re: If a, b, x and y are prime numbers and a^x + b^y = 129, what is the va [#permalink]

Show Tags

New post 15 Sep 2014, 01:56
In fact 5^3+2^2=129 too...thus yielding x+y=5.
Manager
Manager
avatar
G
Joined: 26 Jun 2017
Posts: 248
Location: Russian Federation
Concentration: General Management, Strategy
WE: Information Technology (Other)
Re: If a, b, x and y are prime numbers and a^x + b^y = 129, what is the va [#permalink]

Show Tags

New post 26 Aug 2017, 11:47
thefibonacci wrote:
beef001 wrote:
If a, b, x and y are prime numbers and a^x + b^y = 129, what is the value of x+y?

(A) 5
(B) 6
(C) 10
(D) 13
(E) 15

Can someone explain how to approach this question systematically? Ie. what are your first couple steps in handling this question and how do you break it down?


a^x + b^y = 129 (odd)
all primes except 2 are odd. but if a and b are odd primes then odd+odd = even
so one of a or b has to be 2.
2^x + b^y = 129
2^2 + 5^3 = 129 ;)


Will leave it here).

All prime numbers that can fit: 2, 3, 5, 7, 11.
It is more comfortable to check first the closest to result (129) numbers.

First option fits:
11^2 + 2^3 = 129

I also think there is no systematic approach in such cases, but you can use different prompts to solve them.
In this case yo can use that:
1. One on of them has to be even, because 129 is odd, and all prime numbers are odd except of 2. So we have to have odd + even ---> one of them is 2.
2. 11^2 is the closest to 129, so it is in our best interests to check it first. 2^3 fits. No one saidthat a, b, x, y have to be different.
Re: If a, b, x and y are prime numbers and a^x + b^y = 129, what is the va   [#permalink] 26 Aug 2017, 11:47
Display posts from previous: Sort by

If a, b, x and y are prime numbers and a^x + b^y = 129, what is the va

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

Events & Promotions

PREV
NEXT


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®.