• 5-Day Free Trial
5-day free, full-access trial TTP Quant

Available with Beat the GMAT members only code

• Free Trial & Practice Exam
BEAT THE GMAT EXCLUSIVE

Available with Beat the GMAT members only code

• Magoosh
Study with Magoosh GMAT prep

Available with Beat the GMAT members only code

• Get 300+ Practice Questions

Available with Beat the GMAT members only code

• Free Practice Test & Review
How would you score if you took the GMAT

Available with Beat the GMAT members only code

• Award-winning private GMAT tutoring
Register now and save up to $200 Available with Beat the GMAT members only code • 5 Day FREE Trial Study Smarter, Not Harder Available with Beat the GMAT members only code • 1 Hour Free BEAT THE GMAT EXCLUSIVE Available with Beat the GMAT members only code • Free Veritas GMAT Class Experience Lesson 1 Live Free Available with Beat the GMAT members only code ## (OG-12 DS) Joanna... tagged by: Brent@GMATPrepNow This topic has 1 expert reply and 11 member replies Elena89 Senior | Next Rank: 100 Posts Joined 31 Jul 2011 Posted: 35 messages Followed by: 1 members Thanked: 2 times #### (OG-12 DS) Joanna... Fri Dec 23, 2011 6:31 am Elapsed Time: 00:00 • Lap #[LAPCOUNT] ([LAPTIME]) Joanna bought only$0.15 stamps and $0.29 stamps.How many$0.15 stamps did she buy?

(1) She bought $4.40 worth of stamps. (2) She bought an equal number of$0.15 stamps and $0.29 stamps. OG has a weird explanation for this one. Can someone give a simpler one? Thanks.. Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums! rijul007 Legendary Member Joined 16 Oct 2011 Posted: 588 messages Followed by: 9 members Thanked: 129 times Test Date: 3rd May '12 Target GMAT Score: 750+ GMAT Score: 720 Fri Dec 23, 2011 7:00 am No of$0.15 stamps=x
No of $0.29 stamps=y Quote: (1) She bought$4.40 worth of stamps.
0.15x + 0.29y = 4.4

Not Sufficient

Quote:
(2) She bought an equal number of $0.15 stamps and$0.29 stamps.
x = y

still not sufficient

Combining the two statements

0.15x + 0.29x = 4.4

Find the value of x

Option C

Elena89 Senior | Next Rank: 100 Posts
Joined
31 Jul 2011
Posted:
35 messages
Followed by:
1 members
Thanked:
2 times
Fri Dec 23, 2011 7:35 am
@rijul007

yeah, that's what I did, however that's incorrect!

pemdas Legendary Member
Joined
15 Apr 2011
Posted:
1085 messages
Followed by:
21 members
Thanked:
158 times
Fri Dec 23, 2011 8:05 am
opss in st(2) there's a small nuance

though found in OG-12 it comes from old OGs till now

the question to find integer value of quantity for $0.15 priced stamps bought. let$0.15 priced stamps' quantity be A and $0.29 priced ones' be B, then 0.15A+0.29B=Cost of stamps and we need to find A? st(1) implies Cost Joanna paid was 4.40 and we assume buying B number of$0.29 priced stamps - we must find the possible integer value of B, if any. For this we assign two binomials 0.29B+0.15A=4.40 and assess the values of A and B. If we succeed to find the unique values for A and B, then st(1) is Sufficient, otherwise Not.
B=1 -> 29+15A=440 AND A=(440-29)/15 Not Integer(NI)
B=2 -> 58+15A=440 AND A=(440-58)/15 NI
...
we can use common sense as a number is divisible by 5 if it ends by 0 or 5, and (440-29B) will end in 5 or 0 only if B=5,10,15
B=5, 145+15A=440, A=295/15 NI
B=10, 290+15A=440, A=150/15 good choice
B=15, 435+15A=440, A=5/15 NI

hence we have one unique set when A=10 and B=10 and can answer the question, Sufficient.
check: 0.29*10+0.15*10=4.40

st(2) A=B and we need to know the Cost which is in st(1) only (15A+29B=440 OR 44A=440, A=10) therefore st(2) Alone is Not Sufficient

_________________
Success doesn't come overnight!

Last edited by pemdas on Fri Dec 23, 2011 8:11 am; edited 2 times in total

rijul007 Legendary Member
Joined
16 Oct 2011
Posted:
588 messages
Followed by:
9 members
Thanked:
129 times
Test Date:
3rd May '12
Target GMAT Score:
750+
GMAT Score:
720
Fri Dec 23, 2011 8:09 am
Elena89 wrote:
@rijul007

yeah, that's what I did, however that's incorrect!
ok so its not as easy as it seemed

Statement 1

0.15x + 0.29y = 4.4
or
15x + 29y = 440

Y = 1
15x = 440-29 (not divisible by 15)

y = 5
15x = 440 - 145 (ot divisible by 15)

y = 10

15x = 450 - 290 = 150
x = 10

y = 15 = 145 + 290 = 435

15x = 440 - 29*15 = 440-435
[not divisible by 15]

so no of \$0.15 stamps = 10

Sufficient

Option A

Elena89 Senior | Next Rank: 100 Posts
Joined
31 Jul 2011
Posted:
35 messages
Followed by:
1 members
Thanked:
2 times
Fri Dec 23, 2011 8:15 am
@pemdas

Wrong! The OA is 'A'

What I do not understand is how we can find the value of one unknown from only one equation. What I do understand from OG's explanation is that since both unknowns are 'integers'(whole numbers) therefore from the properties of integer constraints, only one value of any of the 2 unknowns is obtainable! and so the first is sufficient.

pemdas Legendary Member
Joined
15 Apr 2011
Posted:
1085 messages
Followed by:
21 members
Thanked:
158 times
Fri Dec 23, 2011 8:16 am
this q. shows how stupid mistakes may turn our GMAT lives into nightmares, I've mistakenly assumed that the Cost is given following st(1) Sufficient and turned D firstly. Afterwards, seen no word speaks about 4.40 Cost in st(2). Phew

_________________
Success doesn't come overnight!

pemdas Legendary Member
Joined
15 Apr 2011
Posted:
1085 messages
Followed by:
21 members
Thanked:
158 times
Fri Dec 23, 2011 8:18 am
yea, just turned back and seen/corrected/explained in previous post
Elena89 wrote:
@pemdas

Wrong! The OA is 'A'

What I do not understand is how we can find the value of one unknown from only one equation. What I do understand from OG's explanation is that since both unknowns are 'integers'(whole numbers) therefore from the properties of integer constraints, only one value of any of the 2 unknowns is obtainable! and so the first is sufficient.

_________________
Success doesn't come overnight!

Elena89 Senior | Next Rank: 100 Posts
Joined
31 Jul 2011
Posted:
35 messages
Followed by:
1 members
Thanked:
2 times
Fri Dec 23, 2011 8:24 am
yeah, well all that is written in OG too.. but I don't think all that calculation can be done in just 2 minutes..

pemdas Legendary Member
Joined
15 Apr 2011
Posted:
1085 messages
Followed by:
21 members
Thanked:
158 times
Fri Dec 23, 2011 8:37 am
listen, to calculate you need preset values, correct? The complexity and timeliness of your calculation depends on your values. If you start as I and riju from the detail consideration and move onto testing numbers, then yes it's over 2 mins?

However, if you use your number property theory knowledge and apply divisibility by 5 for (440-29B)/15 as I put in my solution, then it's only scratch paper work you do - max. four operations to test st(1) and st(2) is automatically Not Sufficient, unless you as I stupidly follow auto-pilot approach and decide the Cost is given.
Elena89 wrote:
yeah, well all that is written in OG too.. but I don't think all that calculation can be done in just 2 minutes..

_________________
Success doesn't come overnight!

Elena89 Senior | Next Rank: 100 Posts
Joined
31 Jul 2011
Posted:
35 messages
Followed by:
1 members
Thanked:
2 times
Fri Dec 23, 2011 8:52 am
yeah, I get that.. thanks =)

LalaB Master | Next Rank: 500 Posts
Joined
08 Dec 2010
Posted:
425 messages
Followed by:
7 members
Thanked:
56 times
GMAT Score:
690
Fri Dec 23, 2011 10:47 am
Elena89 wrote:
What I do not understand is how we can find the value of one unknown from only one equation.
15x+29y=44

please pay attention to the fact, that x and y must be integers. since the result is too small (44), u should think first about 0, then about 1. so only if x=1 and y=1 we will get 44

stmnt2 is insuf. no info about the sum. all we know that x=y . so (2) is insuf

### GMAT/MBA Expert

Brent@GMATPrepNow GMAT Instructor
Joined
08 Dec 2008
Posted:
10763 messages
Followed by:
1212 members
Thanked:
5144 times
GMAT Score:
770
Fri Dec 23, 2011 11:12 am
LalaB wrote:
Elena89 wrote:
What I do not understand is how we can find the value of one unknown from only one equation.
15x+29y=44

please pay attention to the fact, that x and y must be integers. since the result is too small (44), u should think first about 0, then about 1. so only if x=1 and y=1 we will get 44

stmnt2 is insuf. no info about the sum. all we know that x=y . so (2) is insuf
This is a common trap on the GMAT.

In high school, we learned that we cannot find the value of a variable if we're given 1 equation with 2 variables. However, if we restrict the variables to positive integers, then there are times when we can find the value of a variable if we're given 1 equation with 2 variables.

In this question, the number of each stamp denomination must be a positive integer.

I cover this common GMAT trap (and other common GMAT traps) in video #11 "Avoiding Common Mistakes - Part II." This is a free video you can find at: http://www.gmatprepnow.com/module/gmat-data-sufficiency

Cheers,
Brent

_________________
Brent Hanneson – Founder of GMATPrepNow.com
Use our video course along with

Check out the online reviews of our course
Come see all of our free resources

Thanked by: rijul007, Elena89
GMAT Prep Now's comprehensive video course can be used in conjunction with Beat The GMAT’s FREE 60-Day Study Guide and reach your target score in 2 months!

### Best Conversation Starters

1 Vincen 180 topics
2 lheiannie07 65 topics
3 Roland2rule 49 topics
4 ardz24 40 topics
5 LUANDATO 16 topics
See More Top Beat The GMAT Members...

### Most Active Experts

1 Brent@GMATPrepNow

GMAT Prep Now Teacher

146 posts
2 Rich.C@EMPOWERgma...

EMPOWERgmat

103 posts
3 GMATGuruNY

The Princeton Review Teacher

100 posts
4 EconomistGMATTutor

The Economist GMAT Tutor

92 posts
5 Jay@ManhattanReview

Manhattan Review

79 posts
See More Top Beat The GMAT Experts