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

Redeem
TTP OnDemand GMAT Video Course
Featured Course

Target Test Prep GMAT OnDemand

Scott Woodbury-Stewart’s private virtual classroom — 400 hours of master-class video lessons for the GMAT Focus Edition.

  • 715+ score guarantee — highest in the industry (99th percentile)
  • 52 chapters · 1,500+ lessons · 4,000+ practice questions
  • 400 hours of video · 1,500+ instructor-led HD smartboard lessons
  • 300,000+ students accepted to Harvard, Stanford, Wharton, Booth & Sloan & more
  • 24/7 live support + weekly Zoom office hours with GMAT instructors
  • TTP AI Assist — 24/7 AI-powered virtual tutor for instant help
  • 1,200+ flashcards + AI-powered study assistant & daily calendar
  • OnDemand, LiveTeach & GMAT Bootcamp formats available
  • Also: GRE, SAT Math & Executive Assessment courses
  • MBA Admissions Consulting now available
  • 🏆 2025 EdTech Breakthrough Award: Test Prep Solution Provider of the Year
  • 200,000+ students served
  • 5-day free trial — $0 to start, no auto-billing, cancel anytime
★★★★★ 5.0 (559 reviews)
130-pt guarantee
$0 to start then $127/mo
Start Free 5-Day Trial Read 559 Reviews →

exponenet fun!!

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Master | Next Rank: 500 Posts
Posts: 222
Joined: Tue Mar 25, 2008 3:52 pm
Thanked: 2 times
by vinviper1 » Thu Mar 27, 2008 8:06 pm
What is the sum of the 20th and 21st terms in the sequence 1, 3, 9, 27... where each term is 3 times the preceding term?

1. 4×3*19
2. 10×3*19
3. 4×3*20
4. 10×3*20
5. 339


1

Thanks!
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
Legendary Member
Posts: 645
Joined: Wed Sep 05, 2007 4:37 amFollowed by: 5 members
Location: India
Thanked: 34 times

by camitava » Thu Mar 27, 2008 8:15 pm
This is a GP. To find nth term in GP, the formula is = a * r^(n - 1) where a = first term and r = common ratio. Here a = 1 and r = 3.
So 20th term = 1 * 3^19 = 3^19
So 21st term = 1 * 3^20 = 3^20
Add this two : 3^19(1 + 3) = 3^19 * 4 - So option 1 is the best one to chose!
Correct me If I am wrong


Regards,

Amitava
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Junior | Next Rank: 30 Posts
Posts: 26
Joined: Thu Mar 06, 2008 11:04 am
Thanked: 1 times

by Kaunteya1 » Fri Mar 28, 2008 1:18 pm
Amitava is correct.

1, 3, 9, 27, 81... = 3^0, 3^1, 3^2... The 1st term is 3^0 so the 20th term is 3^19 and the 21st term is 3^20.

3^19 + 3^20 =

3^19(1) + 3^19(3) from step one, basically 3^20 is the same a 3x3x3...(until you have twenty 3's multiplyed together) or 3^20 can be rewritten at 3^19(3) or 3^18(3x3) or 3^17(3x3x3) ect...

So now factor out 3^19 from both 3^19(1) and 3^19(3) this give you
3^19(1 + 3). Just multiply 3^19 back into the () and you get the orignal 3^19 + 3^20.

Finally just add 1+3=4 and then the original 3^19 + 3^20 equals 3^19(4) or A.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 789
Joined: Sun May 06, 2007 1:25 amFollowed by: 6 members
Location: Southern California, USA
Thanked: 15 times

ok

by resilient » Mon Mar 31, 2008 1:18 am
ok so far my approach has been the same but a tad different.

I established the 1 2 9 27 81 243 729 2167 (notice the pattern in units digit.

so the units digitn in th e20th number will end in 7 and 21st number will end in 1 with a units digit of 8 in the final number. Now I just dont understand what the asterik stands for. DOes this mean a secondary multiplication? I hope my approach is right?


please let me know!
Appetite for 700 and I scraped my plate!
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 645
Joined: Wed Sep 05, 2007 4:37 amFollowed by: 5 members
Location: India
Thanked: 34 times

Re: ok

by camitava » Mon Mar 31, 2008 2:12 am
Enginpasa1 wrote:ok so far my approach has been the same but a tad different.

I established the 1 2 9 27 81 243 729 2167 (notice the pattern in units digit.

so the units digitn in th e20th number will end in 7 and 21st number will end in 1 with a units digit of 8 in the final number. Now I just dont understand what the asterik stands for. DOes this mean a secondary multiplication? I hope my approach is right?


please let me know!
Enginpasa, by '*', I tried to mean multiplication. Sorry for the confusion! Look Engin, the main thing I believe is you have to answer the Qs correctly in GMAT in whatever way you follow and that's the main thing.
Correct me If I am wrong


Regards,

Amitava
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Master | Next Rank: 500 Posts
Posts: 222
Joined: Tue Mar 25, 2008 3:52 pm
Thanked: 2 times

by vinviper1 » Mon Mar 31, 2008 10:29 am
Thanks! What do you mean by this being a GP?
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Junior | Next Rank: 30 Posts
Posts: 27
Joined: Thu Apr 10, 2008 1:20 pm
Thanked: 1 times

by humeixia » Thu Apr 10, 2008 2:02 pm
GP?
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 645
Joined: Wed Sep 05, 2007 4:37 amFollowed by: 5 members
Location: India
Thanked: 34 times

by camitava » Thu Apr 10, 2008 5:43 pm
GP means Geometric Progression. Do a google search on this and u will get to know ...
Correct me If I am wrong


Regards,

Amitava
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 789
Joined: Sun May 06, 2007 1:25 amFollowed by: 6 members
Location: Southern California, USA
Thanked: 15 times

I UNDERSTAND

by resilient » Fri Apr 11, 2008 8:56 am
THank you for the heads up Camitava. I have been working on that. I guess the hard part of this question is to learn how to extract the formula out of the way. Where can I learn to how to get better on this!?
Appetite for 700 and I scraped my plate!
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Legendary Member
Posts: 986
Joined: Wed Dec 20, 2006 11:07 amFollowed by: 1 member
Location: India
Thanked: 51 times

Re: I UNDERSTAND

by gabriel » Fri Apr 11, 2008 10:00 am
Enginpasa1 wrote:THank you for the heads up Camitava. I have been working on that. I guess the hard part of this question is to learn how to extract the formula out of the way. Where can I learn to how to get better on this!?
Read the basics on Progressions (a simple google search will give you enough material). Once your done with them, post your doubts on this thread I will try and answer them.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1