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
GMATLiveTeach Starts Oct 17
Chris Peckover, Target Test Prep GMAT expert
LIVE ONLINE CLASSES

Get Ready for GMAT Test Day Faster with Live Online Classes

with Chris Peckover, 100th-Percentile GMAT Scorer

Oct 17 · Chris Peckover
Sat · 11:00 AM to 2:00 PM ET
Oct 20 · Chris Peckover
Tue, Thu · 8:00 to 10:00 PM ET
Oct 25 · Josh Braslow
Sun · 1:00 to 4:00 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
EALiveTeachOnDemand 5 seats left Start anytime
EXECUTIVE ASSESSMENT

Target Test Prep EA OnDemand

Self-paced EA prep. Study on your schedule.

Logan Thompson
EXECUTIVE ASSESSMENT

Sep 6 to Dec 6, 2026

with Logan Thompson

165+ EA score guarantee
$05-day trial no automatic billing
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 Start free 5-day trial
Limited cohort · enrollment openTrial includes full course accessTarget 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.

Sum_progessions

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Legendary Member
Posts: 2326
Joined: Mon Jul 28, 2008 3:54 amFollowed by: 2 members
Thanked: 173 times
GMAT Score:710
by gmatmachoman » Fri May 28, 2010 3:04 am
The sum of the fourth and twelfth term of an arithmetic progression is 20. What is the sum of the first 15 terms of
the arithmetic progression?

A. 300
B. 120
C. 150
D. 170
E. 270
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
Senior | Next Rank: 100 Posts
Posts: 85
Joined: Sat Aug 15, 2009 4:36 am
Thanked: 19 times

by gmatjedi » Fri May 28, 2010 3:17 am
a(n)=a(1)+(n-1)d

a(4)=a(1)+3d
a(12)=a(1)+11d

a(4)+a(12)=20

20=2a(1)+14d

10=a(1)+7d
therefore a(1)=3 and d=1

start sequence at 3...17

sum of first 15 terms= (3+17)/2 * 15= 150
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 576
Joined: Sat Mar 13, 2010 8:31 pmFollowed by: 1 member
Thanked: 97 times

by liferocks » Fri May 28, 2010 4:23 am
T(4)=a+3d
T(12)=a+11d
T(4)+T(12)=2a+14d=20

S(15)=15/2*(2a+14d)=15/2 * 20=150

Ans option C
"If you don't know where you are going, any road will get you there."
Lewis Carroll
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Legendary Member
Posts: 1460
Joined: Tue Dec 29, 2009 1:28 amFollowed by: 7 members
Thanked: 135 times

by selango » Fri May 28, 2010 8:15 am
AP formula =n/2[2a+(n-1)d]


nth term = a+(n-1)d

4th term =a+3d

12th term=a+11d


-->a+3d+a+11d=20
2a+14d=20

Substituting in AP formula with n=15

15/2[2a+14d]


15/2*20

=150
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Senior | Next Rank: 100 Posts
Posts: 56
Joined: Tue Jan 13, 2009 12:59 pm
Location: Toronto
Thanked: 1 times

by odod » Fri May 28, 2010 12:55 pm
Hello.

I need some help understanding this solution.

what is a(n)=a(1)+(n-1)d ?. Is that formula to know for the gmat. If so, can someone please explain it to me?

I would appreciate any help.

Thanks
ODOD
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
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 » Fri May 28, 2010 1:34 pm
gmatmachoman wrote:The sum of the fourth and twelfth term of an arithmetic progression is 20. What is the sum of the first 15 terms of
the arithmetic progression?

A. 300
B. 120
C. 150
D. 170
E. 270
If the sum of the 4th and 12th term is 20, so is the sum of the 1st and 15th. Thus we have 15 terms that have a mean of 20/2 = 10. Sum= 150
Kevin Armstrong
GMAT Instructor
Gmatclasses
Madrid
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Master | Next Rank: 500 Posts
Posts: 165
Joined: Wed Mar 24, 2010 8:02 amFollowed by: 1 member
Thanked: 2 times

by HPengineer » Fri May 28, 2010 1:42 pm
Here a link to the formula used from wikipedia... Might shed some light

https://en.wikipedia.org/wiki/Arithmetic_progression
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1