BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Target Test Prep · GMAT

Choose how you want to prepare

Learn live with an expert or move at your own pace. Every option includes the complete TTP study system.

★★★★★5.0559 reviews
Vote for Target Test Prep, Newsweek Readers’ Choice Awards 2026
NEWSWEEK READERS’ CHOICE 2026

BIG NEWS! Target Test Prep has been nominated, and they’d love your vote!

TTP has worked incredibly hard to build the best test prep experience possible, and winning Newsweek’s 2026 Readers’ Choice Award for Best Test Prep would mean a lot to them. If TTP has helped you, they’d be incredibly grateful for your vote. You can vote once each day through September 9.

Vote for TTP
GMATLiveTeach 7 seats left
Chris Peckover
NEXT LIVE COHORT

Oct 13 to Jan 7, 2027

with Chris Peckover

Schedule
Tue, Thu · 8:00 to 10:00 PM ET
Included
40 live hours + 6 months of GMAT OnDemand
  • Live instruction and real-time questions
  • Class recordings and assigned practice
View class & enroll
Limited cohort · enrollment openTarget Test Prep
EALiveTeach 5 seats left
Logan Thompson
EXECUTIVE ASSESSMENT

Sep 6 to Dec 6, 2026

with Logan Thompson

Schedule
Sun · 9:30 AM to 12:30 PM ET
Included
40 hours of live online classes plus six months of access to the complete TTP EA OnDemand course.
  • 165+ EA Score Guarantee
  • 4,100+ Quant, Verbal, and Integrated Reasoning practice questions
  • 400+ hours of in-depth video lessons
  • 3,000+ step-by-step video solutions
View EA class & enroll
Limited cohort · enrollment openTarget Test Prep
GMATOnDemand Start anytime
SELF-PACED MASTERCLASS

Target Test Prep GMAT OnDemand

Complete access from day one. Study on your schedule.

715+ score guarantee
$0to start then $127/mo
  • Personalized study plan and analytics
  • Thousands of lessons and practice questions
Start free 5-day trial
Read 559 reviewsTarget Test Prep

Compare the format, schedule, and included access before enrolling. Prices and seat counts shown reflect the supplied offer details.

Diagnostic test -- question

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Master | Next Rank: 500 Posts
Posts: 127
Joined: Sat Oct 02, 2010 8:55 amFollowed by: 1 member
Thanked: 17 times
by XLogic » Sat Oct 09, 2010 5:06 pm
Hello All,

I just took the Official GMAT guide Diagnostic test (Math). And boy, I was sweating like a billy-goat!
I really need to get serious about studying.

Anyways, I was wondering if there are any short cuts to answering the question below. The guide does not explain the answer strategy properly.

Question 15. The product of all the prime numbers less than 20 is closest to which of the following powers of 10?

(A) 10^9
(B) 10^8
(C) 10^7
(D) 10^6
(E) 10^5

Thanks for you help.

--Xlogic

PS. I did get the question right, but it took me a long time, some POE and guessing between C and D.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
User avatar
Master | Next Rank: 500 Posts
Posts: 307
Joined: Sun Jul 11, 2010 7:52 pmFollowed by: 1 member
Thanked: 36 times
GMAT Score:640

by limestone » Sat Oct 09, 2010 9:05 pm
My approach:

List all prime numbers in this range:

2,3,5,7,11,13,17,19

Pair each of them:

2*5 = 10

3*7 = 21 = 10*2.1

11*19 = 10^2 * 1.1*1.9

13*17 =10^2 *1.3*1.7

The product will be: 10*10*10^2*10^2 * 2.1*1.1*1.9*1.3*1.7

=10^6 * 2.1*1.1*1.9*1.3*1.7

Take a look at the bold phrase :

1.1*1.9 is near to 2
1.3*1.7 is near to 2
2.1 is near to 2

So their product is aroud 2^3 = 8
Hence the total product is approximately equal to : 8* 10^6
Thus the answer should be 10^7.

Pick C
"There is nothing either good or bad - but thinking makes it so" - Shakespeare.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Master | Next Rank: 500 Posts
Posts: 127
Joined: Sat Oct 02, 2010 8:55 amFollowed by: 1 member
Thanked: 17 times

by XLogic » Sat Oct 09, 2010 10:31 pm
limestone wrote:My approach:

List all prime numbers in this range:

2,3,5,7,11,13,17,19

Pair each of them:

2*5 = 10

3*7 = 21 = 10*2.1

11*19 = 10^2 * 1.1*1.9

13*17 =10^2 *1.3*1.7

The product will be: 10*10*10^2*10^2 * 2.1*1.1*1.9*1.3*1.7

=10^6 * 2.1*1.1*1.9*1.3*1.7

Take a look at the bold phrase :

1.1*1.9 is near to 2
1.3*1.7 is near to 2
2.1 is near to 2

So their product is aroud 2^3 = 8
Hence the total product is approximately equal to : 8* 10^6
Thus the answer should be 10^7.

Pick C
Well done. Thanks.

My approach was a bit crude. In the end I guessed between 10^6 and 10^7. I guess I got lucky.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members
Location: New York, NY
Thanked: 13060 times
GMAT Score:790

by GMATGuruNY » Sun Oct 10, 2010 3:52 am
XLogic wrote:Hello All,

I just took the Official GMAT guide Diagnostic test (Math). And boy, I was sweating like a billy-goat!
I really need to get serious about studying.

Anyways, I was wondering if there are any short cuts to answering the question below. The guide does not explain the answer strategy properly.

Question 15. The product of all the prime numbers less than 20 is closest to which of the following powers of 10?

(A) 10^9
(B) 10^8
(C) 10^7
(D) 10^6
(E) 10^5

Thanks for you help.

--Xlogic

PS. I did get the question right, but it took me a long time, some POE and guessing between C and D.
Since the answer choices are very far apart, we can ballpark the product, and we don't have to be that exact. Every time we round one number down, let's round another number up in order to keep things balanced:

2*3*5*7*11*13*17*19 = ?
2*3*5*7 = 210 ≈ 200
11*13 ≈ 10*15 = 150
17*19 ≈ 15*20 = 300
200*150*300 = 9,000,000

Closest answer is 10,000,000 = 10^7.

The correct answer is C.

Quick and painless!
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Master | Next Rank: 500 Posts
Posts: 127
Joined: Sat Oct 02, 2010 8:55 amFollowed by: 1 member
Thanked: 17 times

by XLogic » Sun Oct 10, 2010 8:18 am
You guys make it look so easy. Thanks for the quick reply.
Now I just need to work on my timing.
It is taking me much too long to complete each section.

Any input is welcome.

Cheers.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Master | Next Rank: 500 Posts
Posts: 268
Joined: Wed Mar 17, 2010 2:32 am
Thanked: 17 times

by this_time_i_will » Sun Oct 10, 2010 10:38 am
we need the value of:
2*3*5*7*11*13*17*19
=210*(10+1)*(10+3)*(10+7)*(10+9)....did a quick multiplication till the point it is easy and got 210.
=210*10*10*10*10 *(2+4+8+10)...took 10 common
=210*24*10*10*10*10
=210 has two 10s and 24 has one 10.
sp total 10s = 2+1+1+1+1+1 = 7
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1