Live GMAT classes with Target Test Prep start Nov 21 and Nov 22. First class free.

See classes
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.

Newsweek Readers’ Choice Awards 2026 winner: Target Test Prep, #1 Test Prep

Target Test Prep was voted #1 Best Test Prep in the 2026 Newsweek Readers’ Choice™ Awards.

★★★★★5.0563 reviews
GMATLiveTeachStarts Nov 22
Josh Braslow, Target Test Prep GMAT expert
LIVE ONLINE CLASSES

Get Ready for GMAT Test Day Faster with Live Online Classes

with Josh Braslow, 20+ years in test prep

Nov 22 · Josh Braslow
Sun · 9:30 AM to 12:30 PM ET
Included
40 hours of live online classes + 6 months of TTP OnDemand
  • Attend the first class for free
  • Every class is recorded, so you never fall behind
View classes & enroll
Limited seats availableTarget Test Prep
EALiveTeachOnDemandStart anytime
EXECUTIVE ASSESSMENT

Target Test Prep EA OnDemand

Self-paced EA prep. Study on your schedule.

EXECUTIVE ASSESSMENT

with

165+ EA score guarantee
$05-day trial no automatic billing
Schedule
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 Start free 5-day trial
Limited cohort · enrollment openTrial includes full course accessTarget Test Prep
GMATOnDemandStart 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 563 reviewsTarget Test Prep

quick comparison

Expert replies
Post new topic Post Reply
Subscribe topic
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 49
Joined: Mon Apr 28, 2008 5:33 am
Thanked: 2 times
by ramyaravindran » Tue Mar 09, 2010 12:02 pm
This may be a simple concept but it is eluding me now. How can you quickly estimate if sqrt(78*79*80*81) is greater than or less than 6400?
Top
Join the discussion
Source: — Problem Solving |
Master | Next Rank: 500 Posts
Posts: 487
Joined: Fri Mar 27, 2009 5:49 am
Thanked: 36 times

by dtweah » Tue Mar 09, 2010 1:46 pm
ramyaravindran wrote:This may be a simple concept but it is eluding me now. How can you quickly estimate if sqrt(78*79*80*81) is greater than or less than 6400?
(Root( 78, 79, 80, 81) )^2


(80 80 )^2

78 78 80 81


80 80 80 80
Top
Join the discussion
GMAT Instructor
Posts: 613
Joined: Thu Mar 22, 2007 6:17 amFollowed by: 64 members
Location: madrid
Thanked: 171 times
GMAT Score:790

by kevincanspain » Tue Mar 09, 2010 2:09 pm
ramyaravindran wrote:This may be a simple concept but it is eluding me now. How can you quickly estimate if sqrt(78*79*80*81) is greater than or less than 6400?
Remember that 81^2 * 79^2 =(80^2 - 1^2)^2 < 80^4

Alternatively, 81 x 79 =80^2 - 1^2 < 80^2

80 x 78 < 80^2
Kevin Armstrong
GMAT Instructor
Gmatclasses
Madrid
Top
Join the discussion
Newbie | Next Rank: 10 Posts
Posts: 4
Joined: Fri Mar 05, 2010 4:09 pm
Thanked: 1 times

by happyzzim » Tue Mar 09, 2010 2:34 pm
Q. sqrt(78*79*80*81) is greater than or less than 6400?

A. sqrt(78*79*80*81) < sqrt(79*79*81*81) = sqrt(79^2*81^2) = 6399 < 6400

For the reason, sqrt(78*79*80*81) < 6400
Top
Join the discussion