Last visit was: 26 Apr 2026, 12:10 It is currently 26 Apr 2026, 12:10
Home
Messages
Tests
Schools
Events
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.
Go to My Error Log Learn more
Close
Request Expert Reply
Confirm Cancel
Back to Forum
Create Topic
555-605 (Medium)|   Algebra|      
User avatar
galiya
User avatar
Joined: 16 Jan 2011
Last visit: 08 Jan 2018
Posts: 72
Own Kudos:
882
 [3]
Given Kudos: 15
Posts: 72
Kudos: 882
 [3]
Does 15a + 7b = 13? 12 Aug 2012, 13:31
1
Kudos
Add Kudos
1
Bookmarks
Bookmark this Post
00:00
Start Timer
Pause Timer
Resume Timer
Show Answer
Hide
Show
History
B
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!

Difficulty:

25% (medium)

Question Stats:

64% (00:55) correct 36% (01:03) wrong based on 88 sessions

History

Date
Time
Result
Not Attempted Yet
Does 15a + 7b = 13?

(1) b = 9

(2) 21b = 39 − 45a

why isnt the 1st statement sufficient?
ShowHide Answer
Official Answer
B
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 26 Apr 2026
Posts: 109,859
Own Kudos:
811,428
Given Kudos: 105,897
Products:
GMAT Club Tests Forum Quiz
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 109,859
Kudos: 811,428
Does 15a + 7b = 13? 12 Aug 2012, 13:35
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Does 15a + 7b = 13?

(1) b = 9. Not sufficient since no info about a.

(2) 21b = 39 − 45a --> re-arrange: 45a+21b=39 --> reduce by 3: 15a+7b=13. Sufficient.

Answer: B.
avatar
mandyrhtdm
avatar
Joined: 05 Jul 2012
Last visit: 09 Mar 2013
Posts: 51
Own Kudos:
177
Given Kudos: 8
Location: India
Concentration: Finance, Strategy
GMAT Date: 09-30-2012
GPA: 3.08
WE:Engineering (Energy)
Posts: 51
Kudos: 177
Does 15a + 7b = 13? 12 Aug 2012, 19:07
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Galiya
Does 15a + 7b = 13?

(1) b = 9

(2) 21b = 39 − 45a

why isnt the 1st statement sufficient?
Show more


b = 9 .. a = ?? what ? as long as you dont know the value of a you cant find the value of 15a + 21b
avatar
bhavinshah5685
avatar
Joined: 25 Jun 2012
Last visit: 19 Jun 2017
Posts: 49
Own Kudos:
321
Given Kudos: 21
Location: India
WE:General Management (Energy)
Posts: 49
Kudos: 321
Does 15a + 7b = 13? 12 Aug 2012, 22:49
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Stmt 1 : if we put the value of b=9 in equation 15a+7b=13, we get the value of a=-10/3
mow again puting value of a=-10/3 and b= 9 in 15a+7b we get 13 as solution.

stmt 2 : sraight forward sufficient.

so, shouldnt the answer be D?
User avatar
EvaJager
User avatar
Joined: 22 Mar 2011
Last visit: 31 Aug 2016
Posts: 513
Own Kudos:
2,371
 [1]
Given Kudos: 43
WE:Science (Education)
Posts: 513
Kudos: 2,371
 [1]
Does 15a + 7b = 13? 12 Aug 2012, 23:07
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
bhavinshah5685
Stmt 1 : if we put the value of b=9 in equation 15a+7b=13, we get the value of a=-10/3
mow again puting value of a=-10/3 and b= 9 in 15a+7b we get 13 as solution.

stmt 2 : sraight forward sufficient.

so, shouldnt the answer be D?
Show more


You don't have the equation \(15a + 7b = 13\) because this is the question itself "Is \(15a + 7b = 13\) ?"
You just have the value of \(b = 9\) and you don't know anything about \(a\), which can be anything.
User avatar
sdpp143
User avatar
Joined: 07 Jun 2012
Last visit: 06 Feb 2018
Posts: 20
Own Kudos:
399
 [1]
Given Kudos: 12
Status:gmat fresher
GPA: 3.87
Posts: 20
Kudos: 399
 [1]
Does 15a + 7b = 13? 17 Aug 2012, 04:29
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
ST1: To get the value of 15a+7b
we should have both a and b
but here only b is given
hence INSUFFICIENT
ST2:21b=39-45a
=>45a+21b=39
=>15a+7b=13
hence SUFFICIENT
so correct ans is B
User avatar
CMcAboy
User avatar
Joined: 01 Mar 2009
Last visit: 10 Apr 2017
Posts: 175
Own Kudos:
116
Given Kudos: 23
Location: United States
Concentration: Strategy, Technology
Schools: Goizueta '16 (M$) Duke '16 (D) Tuck '16 (D) Johnson '16 (D) Darden '16 (D) Ross '16 (WL) McDonough '16 (A)
GMAT 1: 660 Q47 V34
GMAT 2: 680 Q46 V38
GPA: 3.2
WE:Consulting (Computer Software)
Schools: Goizueta '16 (M$) Duke '16 (D) Tuck '16 (D) Johnson '16 (D) Darden '16 (D) Ross '16 (WL) McDonough '16 (A)
GMAT 2: 680 Q46 V38
Posts: 175
Kudos: 116
Does 15a + 7b = 13? 17 Aug 2012, 08:11
Kudos
Add Kudos
Bookmarks
Bookmark this Post
I think Galiya was trying to solve for a.

If the question was:
Given 15a + 7b = 13, what is the value of a + b?
(A) would be sufficient.

However, we are looking for the result of 15a + 7b.
Statement 1 only guys us 15a + 63. Since we don't know the value of a, we cannot find the value of 15a + 63.
Statement 2 gives us an equivalent equation to 15a+7b: 45a+21b=39 -> 15a+7b=13
Since we know the value of 15a + 7b, statement 2 is sufficient.
Answer is (B)
User avatar
mbaiseasy
User avatar
Joined: 13 Aug 2012
Last visit: 29 Dec 2013
Posts: 316
Own Kudos:
2,095
Given Kudos: 11
Concentration: Marketing, Finance
Schools: Full Time MBA (A$)
GPA: 3.23
Schools: Full Time MBA (A$)
Posts: 316
Kudos: 2,095
Does 15a + 7b = 13? 06 Dec 2012, 22:56
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Statement (1) b = 9

15a + 7(9) = 13
15a + 63 = 13
15a = -50
a = -50/15

a could be anything. Thus, INSUFFICIENT!

Statement (2) 45a + 21b = 39

Divide the equation by 3 on both sides: 15a + 7b = 13 Yes! Thus, SUFFICIENT!

Answer: B
User avatar
bumpbot
User avatar
Non-Human User
Joined: 09 Sep 2013
Last visit: 04 Jan 2021
Posts: 38,991
Own Kudos:
1,118
Posts: 38,991
Kudos: 1,118
Does 15a + 7b = 13? 20 Jan 2021, 10:45
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Automated notice from GMAT Club BumpBot:

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

This post was generated automatically.
Moderators:
Bunuel
Math Expert
109859 posts
kingbucky
498 posts
Gmat860sanskar
212 posts