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

 It is currently 23 Oct 2019, 08:57

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

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

(99999)^2-1^2

Author Message
Manager
Joined: 29 May 2008
Posts: 107

Show Tags

Updated on: 17 Jan 2015, 16:42
4
00:00

Difficulty:

15% (low)

Question Stats:

76% (01:04) correct 24% (01:10) wrong based on 200 sessions

HideShow timer Statistics

$$99999^2-1^2$$ =

A) $$10^5$$
B) $$10^{10}$$
C) $$(10^5)(10^5-2)$$
D) $$(10^5)(10-2)$$
E) $$(10^{10})(10^{10}-2)$$

Originally posted by TheRob on 28 Aug 2009, 06:50.
Last edited by Bunuel on 17 Jan 2015, 16:42, edited 1 time in total.
Edited the OA.
Senior Manager
Joined: 18 Aug 2009
Posts: 300
Schools: UT at Austin, Indiana State University, UC at Berkeley
WE 1: 5.5
WE 2: 5.5
WE 3: 6.0

Show Tags

28 Aug 2009, 07:14
1
I am trying to do it, but can not find the answer I got among answers:

9999^2-1^2

(9999+1)(9999-1)
Thus getting 10 0000 (9998).
We can rearrange that as 10^5(10^-2). But there is no such answer in there though
_________________
Never give up,,,
Manager
Joined: 29 May 2008
Posts: 107

Show Tags

28 Aug 2009, 07:55
1
it is C!!!!! thanks your explanation I got to the right path

her it comes

99999^2-1^2

(100000–1)^2–(1^2)=

(10^5)^2 – 2(10^5)+1–1=

(10^5)(10^2–2)
Manager
Joined: 10 Aug 2009
Posts: 96

Show Tags

28 Aug 2009, 08:59
defoue wrote:
TheRob wrote:
it is C!!!!! thanks your explanation I got to the right path

her it comes

99999^2-1^2

(100000–1)^2–(1^2)=

(10^5)^2 – 2(10^5)+1–1=

(10^5)(10^2–2)

Can you pls elaborate how you moved from
(10^5)^2 – 2(10^5)+1–1
to
(10^5)(10^2–2)

It should be $$(10^5)^2-2\times 10^5=10^5\times (10^5-2)$$...I believe it is a mistake...
Manager
Affiliations: CFA Level 2 Candidate
Joined: 29 Jun 2009
Posts: 157
Schools: RD 2: Darden Class of 2012

Show Tags

28 Aug 2009, 09:08
2
This question can be solved the same way as what is $$99^2$$?

Personally I can't do 99x99 in my head. But I can easily do (99x100) - 99 which would give the same result.

Here we do the same thing 99,999x 100,000 - 99,999

= 9,999,900,000 - 99,999 = 9,999,800,001 - now subtract $$1^2$$

All that remains is to figure out the exponents. $$10^5$$ takes care of the 0's and $$10^5-2$$ should take care of the remainder. Unfortunately that doesn't look like any answer given. Are you sure answer choices are correct?

Answer should be $$(10^5)(10^5-2)$$
Manager
Joined: 29 May 2008
Posts: 107

Show Tags

28 Aug 2009, 10:31
Can you pls elaborate how you moved from
(10^5)^2 – 2(10^5)+1–1
to
(10^5)(10^2–2)

well I factorized 10^5 from the terms

(10^5)(10^2 -2 ) + 1 - 1
Manager
Joined: 10 Jul 2009
Posts: 110

Show Tags

28 Aug 2009, 11:11
Manager
Joined: 10 Aug 2009
Posts: 96

Show Tags

28 Aug 2009, 11:27
TheRob wrote:
Can you pls elaborate how you moved from
(10^5)^2 – 2(10^5)+1–1
to
(10^5)(10^2–2)

well I factorized 10^5 from the terms

(10^5)(10^2 -2 ) + 1 - 1

Your factorization is not correct, please see my previous post regarding it.
Manager
Joined: 25 Aug 2009
Posts: 135

Show Tags

28 Aug 2009, 13:24
Answer should be 10^5(10^5 - 2)..

10^5 * 10^2 = 10^7 (not 10^10)

10^5 * 10^5 = 10^10
Manager
Joined: 14 Aug 2009
Posts: 94

Show Tags

29 Aug 2009, 01:08
TheRob wrote:
$$99999^2-1^2$$

$$99999^2-1^2$$
=(99999+1)(99999-1)
=100000*99998
=9999800000
_________________
Kudos me if my reply helps!
Manager
Joined: 29 May 2008
Posts: 107

Show Tags

31 Aug 2009, 06:23
It is true my factorization is wrong

$$99999^2-1^2$$

(99999-1)(99999+1)

(100000-2)(100000)

(10^5 - 2) (10^5)

What do you think?
Intern
Joined: 09 Apr 2013
Posts: 31
Schools: Booth '16

Show Tags

17 Jan 2015, 12:18
EMPOWERgmat Instructor
Status: GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 15315
Location: United States (CA)
GMAT 1: 800 Q51 V49
GRE 1: Q170 V170

Show Tags

17 Jan 2015, 12:31
Hi vedavyas9,

It looks like these posts are from over 5 YEARS ago, so I'm not sure if anyone from this thread is still around. The correct answer IS 10^5(10^5 - 2). Based on the "order" of the answer choices, it seems possible that Answer D just wasn't properly copied (or the original source material had a typo in it) - if the extra "power of 5" were written in, then the correct answer would be D. Maybe Bunuel can edit this?

GMAT assassins aren't born, they're made,
Rich
_________________
Contact Rich at: Rich.C@empowergmat.com

The Course Used By GMAT Club Moderators To Earn 750+

souvik101990 Score: 760 Q50 V42 ★★★★★
ENGRTOMBA2018 Score: 750 Q49 V44 ★★★★★
Intern
Joined: 09 May 2013
Posts: 15

Show Tags

17 Jan 2015, 12:43
1
1
Please Check the options provided. There seems to be some typo

$$99999^2 - 1^2$$
= (99999+1)(99999-1)
(Using the formula $$a^2 - b^2 = (a+b)(a-b)$$)
=$$(100000)(99998)$$
=$$(100000)(100000-2)$$
=$$(10^5)(10^5 - 2)$$
SVP
Status: The Best Or Nothing
Joined: 27 Dec 2012
Posts: 1747
Location: India
Concentration: General Management, Technology
WE: Information Technology (Computer Software)

Show Tags

21 Jan 2015, 03:45
5
$$99999^2 - 1^2$$

$$= (10^5 - 1)^2 - 1$$

$$= 10^{10} - 2*10^5 - 1 + 1$$

$$= (10^5)(10^5-2)$$

_________________
Kindly press "+1 Kudos" to appreciate
Senior Manager
Status: Math is psycho-logical
Joined: 07 Apr 2014
Posts: 402
Location: Netherlands
GMAT Date: 02-11-2015
WE: Psychology and Counseling (Other)

Show Tags

28 Feb 2015, 05:59
I did it sort of differently, but looks simpler to me...:

99999^2 - 1 = (3^2 * 11111) - 1 = 99999 - 1 = 99998

From the answer choices we eliminate A and B right away, and move to C which is:
10^5 - 2 = 100000 - 2 = 99998.

Right...??
Math Expert
Joined: 02 Aug 2009
Posts: 8023

Show Tags

28 Feb 2015, 06:03
1
pacifist85 wrote:
I did it sort of differently, but looks simpler to me...:

99999^2 - 1 = (3^2 * 11111) - 1 = 99999 - 1 = 99998

From the answer choices we eliminate A and B right away, and move to C which is:
10^5 - 2 = 100000 - 2 = 99998.

Right...??

hi pacifist,
you have missed out the power 2 of 99999..
and C is 10^5(10^5-2)
_________________
Senior Manager
Status: Math is psycho-logical
Joined: 07 Apr 2014
Posts: 402
Location: Netherlands
GMAT Date: 02-11-2015
WE: Psychology and Counseling (Other)

Show Tags

28 Feb 2015, 06:05
chetan2u wrote:
pacifist85 wrote:
I did it sort of differently, but looks simpler to me...:

99999^2 - 1 = (3^2 * 11111) - 1 = 99999 - 1 = 99998

From the answer choices we eliminate A and B right away, and move to C which is:
10^5 - 2 = 100000 - 2 = 99998.

Right...??

hi pacifist,
you have missed out the power 2 of 99999..
and C is 10^5(10^5-2)

Yes, I was just realizing exactly that! Also, I very conveniently decided to ignore the first part of answer C. Which worked like charm! Hahahahaha!

Thank you though!
Senior Manager
Joined: 19 Apr 2016
Posts: 268
Location: India
GMAT 1: 570 Q48 V22
GMAT 2: 640 Q49 V28
GPA: 3.5
WE: Web Development (Computer Software)

Show Tags

22 Feb 2017, 23:34
TheRob wrote:
$$99999^2-1^2$$ =

A) $$10^5$$
B) $$10^{10}$$
C) $$(10^5)(10^5-2)$$
D) $$(10^5)(10-2)$$
E) $$(10^{10})(10^{10}-2)$$

$$(a^2-b^2) =(a+b)(a-b)$$

$$99999^2-1^2$$

= (99999-1)(99999+1)

= (100000-2)(100000)

$$= (10^5-2)(10^5)$$

Hence Option C is correct
Hit Kudos if you liked it
Math Expert
Joined: 02 Sep 2009
Posts: 58465

Show Tags

19 Sep 2018, 23:30
TheRob wrote:
$$99999^2-1^2$$ =

A) $$10^5$$
B) $$10^{10}$$
C) $$(10^5)(10^5-2)$$
D) $$(10^5)(10-2)$$
E) $$(10^{10})(10^{10}-2)$$

Discussed here: https://gmatclub.com/forum/99-147049.html

_________________
Re: (99999)^2-1^2   [#permalink] 19 Sep 2018, 23:30
Display posts from previous: Sort by