Digit Reversal

Expert replies

knight247: Legendary Member; Posts: 504; Joined: Tue Apr 19, 2011 1:40 pmFollowed by: 11 members; Thanked: 114 times

by knight247 » Wed Aug 03, 2011 5:27 am

A number B is formed by reversing the two digit number A. What are the digits of A?
(1) Difference of A and B lies between 70 and 80
(2) B is a two digit number

I don't have the OA. Detailed explanations would be appreciated

Like this post

Source: Beat The GMAT — Data Sufficiency | Wed Aug 03, 2011 5:27 am

bblast: Legendary Member; Posts: 1077; Joined: Mon Dec 13, 2010 1:44 amFollowed by: 33 members; Thanked: 118 times; GMAT Score:710

by bblast » Wed Aug 03, 2011 6:13 am

this is a hard prob and I aint a wizard at quant- i'll give it a try :

a=10x + y
b=10y + x

find x,y

stat 1 :

70< 9x-9y <80

less than 8 < x-y < less than 9. {dividing throughout by 9}

since x - y = positive integer therefore x-y = 8

thus,x should be 91 and Y = 19 - sufficient

stat :2
y not equal to 0 - I dont think this is of any use.

answer - A.

I might be hopelessly wrong - but I posted to stand corrected.

Cheers !!

Quant 47-Striving for 50
Verbal 34-Striving for 40

My gmat journey :
https://www.beatthegmat.com/710-bblast-s ... 90735.html
My take on the GMAT RC :
https://www.beatthegmat.com/ways-to-bbla ... 90808.html
How to prepare before your MBA:
https://www.youtube.com/watch?v=upz46D7 ... TWBZF14TKW_

Like this post

Frankenstein: Legendary Member; Posts: 1448; Joined: Tue May 17, 2011 9:55 amFollowed by: 53 members; Location: India; Thanked: 375 times

by Frankenstein » Wed Aug 03, 2011 7:04 am

Hi,
I think bblast got it almost correct.
From(1):
x-y = 8
So, (x,y) can be (9,1) or (8,0)
Not sufficient

From(2):
Not sufficient

Both(1) and (2):
B is two digit number
So, y can't be 0.
So, (x,y) = (9,1)

Hence, C

Can you post the source of this.

Cheers!

Things are not what they appear to be... nor are they otherwise

Like this post

bblast: Legendary Member; Posts: 1077; Joined: Mon Dec 13, 2010 1:44 amFollowed by: 33 members; Thanked: 118 times; GMAT Score:710

by bblast » Wed Aug 03, 2011 8:25 am

Thanks frank- getting q's almost correct wont help me

. But ya this was a bit hard.

I would also like to know the source of this good question.

Like this post

tpr-becky: GMAT Instructor; Posts: 509; Joined: Wed Apr 21, 2010 1:08 pmFollowed by: 85 members; Location: Irvine, CA; Thanked: 199 times; GMAT Score:750

by tpr-becky » Wed Aug 03, 2011 8:39 am

since the difference is between 70 and 80 you know that A must be greater than 70, and that the units digit is probably 0 or 1 (any higher and you are subtracting somethign in the 20's which would result in something less than 70). Try 80 - 08= 72 so that works but so does 91 - 19 = 72. You have two different answers that work so BCE

if B is a two digit number this means that A does not end in 0 - but that doesn't help by itself - CE

But if the two choices from A are 80 and 91 and A can't end in a 0 that means that A must be 91 and the answer is C.

Becky
Master GMAT Instructor
The Princeton Review
Irvine, CA

Like this post

Brian@VeritasPrep: GMAT Instructor; Posts: 1031; Joined: Thu Jul 03, 2008 1:23 pmFollowed by: 255 members; Location: Malibu, CA; Thanked: 716 times; GMAT Score:750

by Brian@VeritasPrep » Wed Aug 03, 2011 8:42 am

Great question with a lot of strategic value - thanks for sharing!

If you didn't think through the lens of algebra you can still attack this one using just arithmetic. You need a difference of at least 70 so you have to start with a number that's 70+ and subtract something that's less than 20. That limits your options...especially if you're thinking of 2-digit numbers forward and backward because then you're subtracting something in the teens.

81 - 18 = 63...not enough
91 - 19 = 72...this one works!
92 - 29 = already too small

So you'd think at first that 91 and 19 is your only combination.

Here's where Data Sufficiency strategy comes in. Statement 2 alone looks pretty useless. I sort of already assumed that B was two digits, so why are they telling me that? If you read those pretty bland/useless statements by thinking "Why Are You Here?", they often tell you something really important - in this case, it's telling me that I don't have to use a number B that's in the teens...I could use a single digit number like 08 or 09. Well, that allows for 80-08 = 72 - another possibility that shows me that statement 2 alone is not sufficient. I need both to finally arrive at 91-19. So the answer is C, and I learned that by trying to determine why statement 2 even existed.

Here, the role of statement 2 is to make explicit a rule that most of us probably assumed in the first place. In events like that, you need to ask yourself whether that rule needed to be made explicit or not. And going back to the question, the only thing we know is that we reversed 2-digit number A...we don't know at all that B remains at two digits until statement 2 explicitly defines that.

Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep

Looking for GMAT practice questions? Try out the Veritas Prep Question Bank. Learn More.

Like this post

y_roy82: Junior | Next Rank: 30 Posts; Posts: 14; Joined: Sat Nov 20, 2010 10:19 pmFollowed by: 1 member

by y_roy82 » Fri Aug 05, 2011 9:46 am

Brian@VeritasPrep wrote:Great question with a lot of strategic value - thanks for sharing!

Here's where Data Sufficiency strategy comes in. Statement 2 alone looks pretty useless. I sort of already assumed that B was two digits, so why are they telling me that? If you read those pretty bland/useless statements by thinking "Why Are You Here?", they often tell you something really important - in this case, it's telling me that I don't have to use a number B that's in the teens...I could use a single digit number like 08 or 09. Well, that allows for 80-08 = 72 - another possibility that shows me that statement 2 alone is not sufficient. I need both to finally arrive at 91-19. So the answer is C, and I learned that by trying to determine why statement 2 even existed.

Aren't we reading the question incorrectly here
1) statement ambiguous... says difference of i.e., 70 < |A-B| < 80
if this is correct then from 1) & 2) A could be 19 or 91.

Further more the question asks what are the digits of A?
in general someone will answer 1 & 9 or one may say 9 & 1. (place value not given importance here )

so answer is C. I don't think A=91

correct me if I am wrong...

i used this general method to solve, much easier than trying numbers i think
x and y = digits
so
A = 10X + Y
B = 10Y + X

1) 70 < difference < 80
|A-B| = 9|X-Y|
therefore, 70 < 9|X-Y| < 80
gives X-Y = 8
possible values 9,1 or 8,0
both satisfy so INSUFFICIENT

2) B is 2 digit number
INSUFFICIENT

1) & 2) together
8,0 = get out mate
only remains 1,9
so answer C

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review