It is currently 18 Jan 2018, 07:39

### 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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# M15-07

Author Message
TAGS:

### Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 43322

Kudos [?]: 139397 [1], given: 12789

### Show Tags

15 Sep 2014, 23:55
1
KUDOS
Expert's post
1
This post was
BOOKMARKED
00:00

Difficulty:

45% (medium)

Question Stats:

67% (00:51) correct 33% (00:48) wrong based on 70 sessions

### HideShow timer Statistics

Which of the following is equal to $$(0.9998)^2$$?

A. 0.99950014
B. 0.99950234
C. 0.99960004
D. 0.99960064
E. 0.99961024
[Reveal] Spoiler: OA

_________________

Kudos [?]: 139397 [1], given: 12789

Math Expert
Joined: 02 Sep 2009
Posts: 43322

Kudos [?]: 139397 [1], given: 12789

### Show Tags

15 Sep 2014, 23:55
1
KUDOS
Expert's post
Official Solution:

Which of the following is equal to $$(0.9998)^2$$?

A. 0.99950014
B. 0.99950234
C. 0.99960004
D. 0.99960064
E. 0.99961024

$$0.9998^2 = (0.9998 - 0.0002)(0.9998 + 0.0002) + (0.0002)^2 =$$

$$=0.9996*1 + 0.00000004 = 0.99960004$$.

_________________

Kudos [?]: 139397 [1], given: 12789

Current Student
Joined: 08 Feb 2014
Posts: 205

Kudos [?]: 84 [0], given: 145

Location: United States
Concentration: Finance
GMAT 1: 650 Q39 V41
WE: Analyst (Commercial Banking)

### Show Tags

20 Oct 2014, 17:34
What is the rationale for the above? I'm not understanding the idea of subtracting .0002, and then adding it back to the other factor.

While solving, I just noticed that the second digit from the right, in each of the answer choices was different, so we only need to multiply the "9" and "8" terms across .9998, add the resulting lines, and see which choice matched the second digit in the answer we just found.

Thanks

Last edited by JackSparr0w on 23 Oct 2014, 06:50, edited 1 time in total.

Kudos [?]: 84 [0], given: 145

Intern
Joined: 16 Oct 2014
Posts: 9

Kudos [?]: 10 [1], given: 1

### Show Tags

20 Oct 2014, 22:16
1
KUDOS
I think this is quickest to work out using good old long mulptiplication-
We only need to work out the second last digit as these are all different
9998*9998 =
79984
89982

So this will sum to a number ending with 04 which is option C

EDIT *
Not able to get the allignments right on the long multiplication so this is really hard to understand for anyone who do not understand long multiplication. Anyone able to fix it?

Kudos [?]: 10 [1], given: 1

Intern
Joined: 22 Nov 2012
Posts: 22

Kudos [?]: 7 [0], given: 90

Location: United States

### Show Tags

28 Dec 2015, 11:29
I think this the explanation isn't clear enough, please elaborate.
_________________

GMAT,
It is not finished untill I win!!!

Kudos [?]: 7 [0], given: 90

Current Student
Joined: 22 Dec 2015
Posts: 98

Kudos [?]: 19 [0], given: 40

Concentration: General Management
GMAT 1: 760 Q48 V47
GPA: 3.89
WE: Accounting (Energy and Utilities)

### Show Tags

28 Dec 2015, 11:43
Yup, I just multiplied the last two digits as well. Takes like 30 seconds!

Kudos [?]: 19 [0], given: 40

Intern
Joined: 22 Nov 2012
Posts: 22

Kudos [?]: 7 [0], given: 90

Location: United States

### Show Tags

28 Dec 2015, 13:39
I see. But I was refering to Bunuel's explanation. I also ended up in moltiplying the last two terms but I got D instead of C.
Can anyone explain Bunuel's method? It might be useful for the next time I face such a question.

Thanks!
_________________

GMAT,
It is not finished untill I win!!!

Kudos [?]: 7 [0], given: 90

Intern
Joined: 12 Feb 2016
Posts: 8

Kudos [?]: 1 [0], given: 0

### Show Tags

11 Jul 2016, 01:56
1
This post was
BOOKMARKED
The easiest way is to rewrite the decimal as a binomial and then apply the rule for the square of a binomial $$(a+b)^2$$ = $$a^2+b^2+2ab$$
therefore $$(0.9998)^2$$ = $$(1-2*10^-4)^2$$ do the math and find the answer.

Kudos [?]: 1 [0], given: 0

Manager
Joined: 14 Oct 2012
Posts: 180

Kudos [?]: 63 [0], given: 966

### Show Tags

30 Mar 2017, 19:56
My 2 cents:
(0.9998)^2 = (1.0000 - 0.0002)^2 = (1.0000)^2 - 2*(1.0000)*(0.0002) + (0.0002)^2 = 1 - 0.0004 + 0.00000004
= 0.9996 + 0.00000004 = 0.99960004 | C
(a-b)^2 = a^2 - 2*a*b + b^2

Kudos [?]: 63 [0], given: 966

Intern
Joined: 21 Jul 2015
Posts: 15

Kudos [?]: 1 [0], given: 33

### Show Tags

17 Sep 2017, 10:19
Just take last 2 digits, which are 9 and 8. So 98 * 98. The answer is 9604. We dnt need to even complete this 98*98 calculation. Look at last 2 digits of the end result, 04. There is only one option providing 04.
_________________

NOW OR NEVER

Kudos [?]: 1 [0], given: 33

Re: M15-07   [#permalink] 17 Sep 2017, 10:19
Display posts from previous: Sort by

# M15-07

Moderators: chetan2u, Bunuel

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.