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

Redeem

Sequence (GPREP)

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Legendary Member
Posts: 1169
Joined: Sun Jul 06, 2008 2:34 am
Thanked: 25 times
Followed by:1 members

Sequence (GPREP)

by aj5105 » Tue Jun 16, 2009 11:41 pm
Given a sequence a1, a2, a3……. a15

In the sequence shown, an = an-1 + k, where 2 =< n =<15 and k is a non-zero constant. How many of the terms in the sequence are greater than 10?

(1) a1 = 24
(2) a8 = 10
Top
Source: — Data Sufficiency |
Master | Next Rank: 500 Posts
Posts: 418
Joined: Wed Jun 11, 2008 5:29 am
Thanked: 65 times

by bluementor » Tue Jun 16, 2009 11:52 pm
an = an-1 + k, where 2 =< n =<15 and k is a non-zero constant

How many of the terms in the sequence are greater than 10?

Statement 1: a1 = 24

Since we dont know k, we cannot know how many terms will be greater than 10.

for eg.
-if k > 0, then all 15 terms will be greater than 10.
-if k = -20, then only the first term, a1, is greater than 10.

Insufficient.

Statement 2: a8 = 10

Notice that a8 is the middle term of the 15-term sequence. So this means there are 7 terms greater than 10, and 7 terms smaller than 10 (notice that this true regardless of what k is, as long as k is nonzero). We have sufficient info to answer this question.

Choose B.

-BM-
Top
Legendary Member
Posts: 1169
Joined: Sun Jul 06, 2008 2:34 am
Thanked: 25 times
Followed by:1 members

by aj5105 » Wed Jun 17, 2009 12:19 am
I have a couple of questions here. They may be stupid, but please bear with them.

1.Are the terms in a sequence always arranged in ascending order?

2.Is there a possibility that all terms from a8 to a15 remain same?
Top
Junior | Next Rank: 30 Posts
Posts: 12
Joined: Mon Jun 15, 2009 8:59 pm

by shashank.mehra » Wed Jun 17, 2009 12:31 am
aj5105 wrote:I have a couple of questions here. They may be stupid, but please bear with them.

1.Are the terms in a sequence always arranged in ascending order?

2.Is there a possibility that all terms from a8 to a15 remain same?
1. It depends whether the common difference (in this case case K is common differnce) is positive or negative

2. As K is non-zero therefore, either the terms will be in ascending (if k is +ve) or descending (if k is -ve)
Top
Legendary Member
Posts: 752
Joined: Sun May 17, 2009 11:04 pm
Location: Tokyo
Thanked: 81 times
GMAT Score:680

by tohellandback » Wed Jun 17, 2009 12:37 am
aj5105 wrote:I have a couple of questions here. They may be stupid, but please bear with them.

1.Are the terms in a sequence always arranged in ascending order?

2.Is there a possibility that all terms from a8 to a15 remain same?
aj5105,
I agree with bluementor about the answer B
as for your questions:
1) No, a sequence is not always in an order. It depends upon the sequence's formula or function. Arithmetic or geometric progression are just examples of sequence

2) It is possible but not in this case. it is clearly mentioned that K is a non-zero constant. if K can be zero then of course all the terms are equal
The powers of two are bloody impolite!!
Top
Junior | Next Rank: 30 Posts
Posts: 21
Joined: Fri Nov 21, 2008 9:41 am
Thanked: 1 times

by Umar82 » Wed Aug 19, 2009 2:56 pm
is there a typo in this question? how can there be 15 sequences if 2 ≤ n ≤ 15 ??
Top
User avatar
Legendary Member
Posts: 758
Joined: Sat Aug 29, 2009 9:32 pm
Location: Bangalore,India
Thanked: 67 times
Followed by:2 members

by sumanr84 » Mon Jul 19, 2010 7:52 am
aj5105 wrote:Given a sequence a1, a2, a3��. a15

In the sequence shown, an = an-1 + k, where 2 =< n =<15 and k is a non-zero constant. How many of the terms in the sequence are greater than 10?

(1) a1 = 24
(2) a8 = 10
Tricky question..almost got me..worth solving...
[spoiler]
takeaway : C-Trap says " In DS, Always be suspicious of answers that you can get using 2 options without using your pen i.e. too obvious looking answer." - This takeaway saved me in this question.[/spoiler]
Top
User avatar
Legendary Member
Posts: 1893
Joined: Sun May 30, 2010 11:48 pm
Thanked: 215 times
Followed by:7 members

by kvcpk » Mon Jul 19, 2010 8:10 am
Given a sequence a1, a2, a3??. a15

In the sequence shown, an = an-1 + k, where 2 =< n =<15 and k is a non-zero constant. How many of the terms in the sequence are greater than 10?

(1) a1 = 24

a1= 24
a2=24+k
a3=24+2k
.....
We cannot say how many are greater than 10, because, we do not know if K is positive or negative.

(2) a8 = 10
Series is linear.. Whether it needs to increase or decrease linearly.
a1,a2,..........a8,..............a14,a15
On either side of a8, there are only 7 terms.
So either a1-a7 or a9-a15 need to be less than 10.

hence pick B.

I was almost getting it wrong when I looked at why 2<=n<=15 is given. That made ot easy for me.
Top
Senior | Next Rank: 100 Posts
Posts: 95
Joined: Wed Mar 10, 2010 3:23 am
Thanked: 1 times
Followed by:1 members

by clammiestqasar » Mon Aug 02, 2010 12:18 am
we dont have 7 terms on either side of a8.Thereare 6 terms on the left of a8 and 7 to the right
2,3,4,5,6,7
9,10,11,12,13,14,15
Top
User avatar
Legendary Member
Posts: 1893
Joined: Sun May 30, 2010 11:48 pm
Thanked: 215 times
Followed by:7 members

by kvcpk » Mon Aug 02, 2010 12:33 am
clammiestqasar wrote:we dont have 7 terms on either side of a8.Thereare 6 terms on the left of a8 and 7 to the right
2,3,4,5,6,7
9,10,11,12,13,14,15
The series is given as a1,a2,a3...
Top
Senior | Next Rank: 100 Posts
Posts: 95
Joined: Wed Mar 10, 2010 3:23 am
Thanked: 1 times
Followed by:1 members

by clammiestqasar » Mon Aug 02, 2010 2:06 am
yea..apologies for being so stupid.
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 3380
Joined: Mon Mar 03, 2008 1:20 am
Thanked: 2256 times
Followed by:1535 members
GMAT Score:800

by lunarpower » Tue Aug 03, 2010 3:23 am
Umar82 wrote:is there a typo in this question? how can there be 15 sequences if 2 ≤ n ≤ 15 ??
the problem is not stating that terms only exist for 2 ≤ n ≤ 15; rather, the problem is stating that the given rule for generating terms only works when 2 ≤ n ≤ 15.
since the rule in question refers to a(n-1) and a(n), it actually implies the existence of, at the very least, terms for 1 ≤ n ≤ 15 (since plugging in n = 2 requires the existence of terms for n = 1 and n = 2).
Ron has been teaching various standardized tests for 20 years.

--

Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi

--

Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.

Yves Saint-Laurent

--

Learn more about ron
Top
Post new topic Post Reply
• Page 1 of 1