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 →

series

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Newbie | Next Rank: 10 Posts
Posts: 9
Joined: Mon Oct 26, 2009 10:55 pm
Thanked: 1 times
GMAT Score:670
by kaustubh_b » Sat Feb 13, 2010 6:53 am
This one is from GMAT Prep.
If each term of the series a1 + a2 + ... + an is either 7 or 77 and the sum equals 350, which of the following could be equal to n?

(A)38
(B)39
(C)40
(D)41
(E)42

The answer in this case is C and I was able to solve using substitution.
However, it would be great to know if there are any simpler and better approaches to this problem.

Thanks in advance.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
User avatar
Legendary Member
Posts: 1275
Joined: Thu Sep 21, 2006 11:13 pmFollowed by: 2 members
Location: Arabian Sea
Thanked: 125 times

by ajith » Sat Feb 13, 2010 7:12 am
kaustubh_b wrote:This one is from GMAT Prep.
If each term of the series a1 + a2 + ... + an is either 7 or 77 and the sum equals 350, which of the following could be equal to n?

(A)38
(B)39
(C)40
(D)41
(E)42

The answer in this case is C and I was able to solve using substitution.
However, it would be great to know if there are any simpler and better approaches to this problem.

Thanks in advance.
If each term of the series a1 + a2 + ... + an is either 7 or 77
If each term of the series a1/7 + a2 /7+ ... + an/7 is either 1 or 11

a1/7 + a2 /7+ ... + an/7 =50

Now if one of the terms is 11 and all others 1s ; no of terms = 39 (1's) + 1 = 40
if two of the terms are 11 and all others 1s ; no of terms = 28 (1's) + 2 = 30
if Three of the terms are 11 and all others 1s ; no of terms = 17 (1's) + 3 = 20
if four of the terms are 11 and all others 1s ; no of terms = 6 (1's) + 4 = 10

These are the only possibilities 40 is given in the choices - hence C
Always borrow money from a pessimist, he doesn't expect to be paid back.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Master | Next Rank: 500 Posts
Posts: 212
Joined: Sat Dec 01, 2007 4:19 pm
Thanked: 5 times

by vscid » Sat Feb 13, 2010 10:12 am
This is how I did it, 42 secs.

7x + 77y = 350
x+ y = n

n can have more than 1 values, but in order to maximize n, x has to be maximized.
This is possible when y = 1. So, when y = 1, x = 39.
Hence C.
The GMAT is indeed adaptable. Whenever I answer RC, it proficiently 'adapts' itself to mark my 'right' answer 'wrong'.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
GMAT Instructor
Posts: 3225
Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members
Location: Toronto
Thanked: 1710 times
GMAT Score:800

by Stuart@KaplanGMAT » Sat Feb 13, 2010 5:15 pm
kaustubh_b wrote:This one is from GMAT Prep.
If each term of the series a1 + a2 + ... + an is either 7 or 77 and the sum equals 350, which of the following could be equal to n?

(A)38
(B)39
(C)40
(D)41
(E)42

The answer in this case is C and I was able to solve using substitution.
However, it would be great to know if there are any simpler and better approaches to this problem.

Thanks in advance.
15 second solution: if each term ends in 7, and the sum of the terms ends in 0, then the number of terms MUST be a multiple of 10... choose (C)!
Image

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Master | Next Rank: 500 Posts
Posts: 212
Joined: Sat Dec 01, 2007 4:19 pm
Thanked: 5 times

by vscid » Sat Feb 13, 2010 5:35 pm
Stuart Kovinsky wrote:
kaustubh_b wrote:This one is from GMAT Prep.
If each term of the series a1 + a2 + ... + an is either 7 or 77 and the sum equals 350, which of the following could be equal to n?

(A)38
(B)39
(C)40
(D)41
(E)42

The answer in this case is C and I was able to solve using substitution.
However, it would be great to know if there are any simpler and better approaches to this problem.

Thanks in advance.
15 second solution: if each term ends in 7, and the sum of the terms ends in 0, then the number of terms MUST be a multiple of 10... choose (C)!
Damn ! missed it!
The GMAT is indeed adaptable. Whenever I answer RC, it proficiently 'adapts' itself to mark my 'right' answer 'wrong'.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1