Last visit was: 13 Jul 2025, 20:30 It is currently 13 Jul 2025, 20:30
Close
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Close
Request Expert Reply
Confirm Cancel
User avatar
Gnpth
Joined: 29 Aug 2012
Last visit: 03 Mar 2023
Posts: 1,049
Own Kudos:
6,616
 [13]
Given Kudos: 330
Status:Chasing my MBB Dream!
Location: United States (DC)
WE:General Management (Aerospace and Defense)
Products:
Posts: 1,049
Kudos: 6,616
 [13]
1
Kudos
Add Kudos
11
Bookmarks
Bookmark this Post
User avatar
BrentGMATPrepNow
User avatar
Major Poster
Joined: 12 Sep 2015
Last visit: 13 May 2024
Posts: 6,755
Own Kudos:
34,075
 [2]
Given Kudos: 799
Location: Canada
Expert
Expert reply
Posts: 6,755
Kudos: 34,075
 [2]
2
Kudos
Add Kudos
Bookmarks
Bookmark this Post
User avatar
stne
Joined: 27 May 2012
Last visit: 13 Jul 2025
Posts: 1,768
Own Kudos:
Given Kudos: 656
Posts: 1,768
Kudos: 1,839
Kudos
Add Kudos
Bookmarks
Bookmark this Post
User avatar
IanStewart
User avatar
GMAT Tutor
Joined: 24 Jun 2008
Last visit: 13 Jul 2025
Posts: 4,141
Own Kudos:
10,620
 [3]
Given Kudos: 97
 Q51  V47
Expert
Expert reply
Posts: 4,141
Kudos: 10,620
 [3]
2
Kudos
Add Kudos
1
Bookmarks
Bookmark this Post
As Brent says above, you don't need to know anything about "geometric series" for the GMAT - you don't even need to know what they are. So ignoring the other problems with the wording of the question, it's simply out of the scope of the test. The only way it could appear is if the question itself also provided the formulas you'd need to use to answer it.

But since the only solution above assumes r is a positive integer, and uses inspection, I can offer a different method, though test takers can safely ignore questions like this. We know from Statement 1 that the first term a = 1, and the last term is 1024. In a geometric sequence with n terms, the last term is equal to (a)(r^(n-1)). Since a =1, we know

r^(n-1) = 1024

The sum S of a geometric sequence with n terms is equal to

S = a(1- r^n)/(1 - r)

so since that sum is 2047, we also know, using that a=1,

2047 = (1 - r^n) / (1 - r)

Since r^n is just equal to (r)(r^(n-1)) by basic exponent rules, and since r^(n-1) = 1024, we can replace r^n with 1024r. So

2047 = (1 - 1024r) / (1 - r)
2047 - 2047r = 1 - 1024r
2046 = 1023r
2 = r

and Statement 1 is sufficient. Statement 2 is not sufficient (it could be the sequence with r=2 that we found in Statement 1, but since r need not be an integer, it could just be a two-term sequence where the first term is 512, and the second is 1535, among many possibilities), so the answer is A.
avatar
chandra73
Joined: 24 Dec 2019
Last visit: 24 Nov 2020
Posts: 3
Given Kudos: 116
Posts: 3
Kudos: 0
Kudos
Add Kudos
Bookmarks
Bookmark this Post
BrentGMATPrepNow
Gnpth
The sum of the terms of a geometric progression is 2047. Find the common ratio.

(1) The first and last terms of the series are 1 and 1024 respectively.

(2) Last but one term of the series is 512.

In case there are people reading this question and worrying that they haven't learned about geometric progressions and common ratios, you need not worry. The GMAT does not expect you to be familiar with these terms. Likewise, you don't need to know the formula for the sum of a geometric series/sequence/progression.

Cheers,
Brent

Thank you Brent for Clarification!
avatar
Samuel0709
avatar
School Moderator - LBS Masters
Joined: 24 Jun 2021
Last visit: 22 Nov 2024
Posts: 52
Own Kudos:
Given Kudos: 37
Status:One day at a time
Location: India
GMAT 1: 540 Q47 V19
GMAT 2: 630 Q49 V27
GMAT 3: 710 Q49 V39
WE:Manufacturing and Production (Manufacturing)
GMAT 3: 710 Q49 V39
Posts: 52
Kudos: 24
Kudos
Add Kudos
Bookmarks
Bookmark this Post
To solve this problem we just need to know the formula that defines the nth term in a G.P :
An= A1 R^(n-1)
Given an=1024 and a1=1
=> 1024= 1 x R^(n-1)
=> 1024= R^(n-1)
=> for a given sum of the sequence we can find R. Hence Statement one is sufficient.
Statement two merely tells us the second last element in the sequence is 512. Without knowing the last element we cannot conclude on what the common ratio might be
User avatar
bumpbot
User avatar
Non-Human User
Joined: 09 Sep 2013
Last visit: 04 Jan 2021
Posts: 37,378
Own Kudos:
Posts: 37,378
Kudos: 1,010
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
Moderator:
Math Expert
102639 posts