GMAT Changed on April 16th - Read about the latest changes here

 It is currently 24 May 2018, 22:26

### 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

# M09-14

Author Message
TAGS:

### Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 45373

### Show Tags

16 Sep 2014, 00:39
1
KUDOS
Expert's post
1
This post was
BOOKMARKED
00:00

Difficulty:

25% (medium)

Question Stats:

78% (00:55) correct 22% (03:11) wrong based on 91 sessions

### HideShow timer Statistics

What is 987 * 987?

A. 974,169
B. 974,219
C. 974,549
D. 975,019
E. 975,369

_________________
Math Expert
Joined: 02 Sep 2009
Posts: 45373

### Show Tags

16 Sep 2014, 00:39
2
KUDOS
Expert's post
3
This post was
BOOKMARKED
Official Solution:

What is 987 * 987?

A. 974,169
B. 974,219
C. 974,549
D. 975,019
E. 975,369

$$987*987 = (987 + 13)*(987 - 13) + 13^2 = 1000*974 + 169 = 974,169$$. We used $$(a + b)(a - b) + b^2 = a^2$$.

_________________
Manager
Joined: 08 Feb 2014
Posts: 205
Location: United States
Concentration: Finance
GMAT 1: 650 Q39 V41
WE: Analyst (Commercial Banking)

### Show Tags

04 Nov 2014, 17:59
1
KUDOS
Notice that the 10's digit of all the answer choices is different. Perform regular multiplication till you can calculate what the 10's digit will be.

987
x987
6906 (multiplied 7 in denominator by the numerator)
78960 (multiplied numerator by 8)

No need to multiply by the 9 in the denominator. We can see that the tens digit will be 6. We actually didn't need to multiply the 7 or the 8 all the way across either, but it's simple to do, and may help avoid errors.

Best
Intern
Joined: 12 Mar 2015
Posts: 46
Schools: Haas '20
GPA: 2.99
WE: Corporate Finance (Aerospace and Defense)

### Show Tags

17 Mar 2016, 09:17
Thanks Brunnel! That was a great breakdown of using the difference of two squares to solve this problem!
Intern
Joined: 09 Jul 2016
Posts: 17
GMAT 1: 730 Q50 V39

### Show Tags

24 May 2017, 03:59
I think this is a high-quality question. Another approach - 987 x 987 = (1000-13) x (1000-13) = (1000-13)^2
Intern
Joined: 01 Aug 2017
Posts: 5

### Show Tags

30 Jan 2018, 03:51
1
KUDOS
The best way to solve this question (What is 987 * 987?) is by elimination method.

we know 987*987 has to be divisible by 9, since 987 is divisible by 3.
Using the divisor rule of 9 in the options: correct answer should be the one whose sum of digits is divisible by 9.

974,169= 36
974,219= 32
974,549= 38
975,019= 31
975,369= 39

only 36 is divisible by 9, so A is correct
Re: M09-14   [#permalink] 30 Jan 2018, 03:51
Display posts from previous: Sort by

# M09-14

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®.