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

Available with Beat the GMAT members only code

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

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

• Get 300+ Practice Questions

Available with Beat the GMAT members only code

• 1 Hour Free
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

• Free Trial & Practice Exam
BEAT THE GMAT EXCLUSIVE

Available with Beat the GMAT members only code

• 5 Day FREE Trial
Study Smarter, Not Harder

Available with Beat the GMAT members only code

• Award-winning private GMAT tutoring

(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! pemdas Legendary Member Joined 15 Apr 2011 Posted: 1085 messages Followed by: 21 members Upvotes: 158 Top Reply 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 Upvotes: 2 Top Reply 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 Upvotes: 158 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 Upvotes: 2 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.. ### GMAT/MBA Expert Brent@GMATPrepNow GMAT Instructor Joined 08 Dec 2008 Posted: 11041 messages Followed by: 1217 members Upvotes: 5254 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 â€“ 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 GMATâ€™s FREE 60-Day Study Guide and reach your target score in 2 months! LalaB Master | Next Rank: 500 Posts Joined 08 Dec 2010 Posted: 425 messages Followed by: 7 members Upvotes: 56 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 Elena89 Senior | Next Rank: 100 Posts Joined 31 Jul 2011 Posted: 35 messages Followed by: 1 members Upvotes: 2 Fri Dec 23, 2011 8:52 am yeah, I get that.. thanks =) pemdas Legendary Member Joined 15 Apr 2011 Posted: 1085 messages Followed by: 21 members Upvotes: 158 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! pemdas Legendary Member Joined 15 Apr 2011 Posted: 1085 messages Followed by: 21 members Upvotes: 158 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! Elena89 Senior | Next Rank: 100 Posts Joined 31 Jul 2011 Posted: 35 messages Followed by: 1 members Upvotes: 2 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. rijul007 Legendary Member Joined 16 Oct 2011 Posted: 588 messages Followed by: 9 members Upvotes: 130 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! The correct answer is A 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

pemdas Legendary Member
Joined
15 Apr 2011
Posted:
1085 messages
Followed by:
21 members
158
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

### Best Conversation Starters

1 lheiannie07 116 topics
2 LUANDATO 67 topics
3 swerve 66 topics
4 ardz24 61 topics
5 AAPL 59 topics
See More Top Beat The GMAT Members...

### Most Active Experts

1 Scott@TargetTestPrep

Target Test Prep

213 posts
2 Brent@GMATPrepNow

GMAT Prep Now Teacher

177 posts
3 Jeff@TargetTestPrep

Target Test Prep

168 posts
4 Rich.C@EMPOWERgma...

EMPOWERgmat

133 posts
5 GMATGuruNY

The Princeton Review Teacher

126 posts
See More Top Beat The GMAT Experts