Difficult Math Question #27 - Series - The Beat The GMAT Forum - Expert GMAT Help & MBA Admissions Advice

Difficult Math Question #27 - Series

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800guy: Master | Next Rank: 500 Posts; Posts: 354; Joined: Tue Jun 27, 2006 9:20 pmFollowed by: 5 members; Thanked: 11 times

by 800guy » Thu Oct 12, 2006 10:10 am

OA coming after some people answer

Find the fourth consecutive even number:
(I) the sum of the last two numbers is 30
(II) the sum of the first two numbers is 22

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Source: Beat The GMAT — Data Sufficiency | Thu Oct 12, 2006 10:10 am

countingdolls: Junior | Next Rank: 30 Posts; Posts: 13; Joined: Mon Oct 02, 2006 7:07 am

by countingdolls » Thu Oct 12, 2006 12:02 pm

(II) is enough, (I) isnt...so, B

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abby_g: Junior | Next Rank: 30 Posts; Posts: 24; Joined: Sat Sep 30, 2006 1:59 am; Location: Mumbai; Thanked: 1 times

by abby_g » Fri Oct 13, 2006 6:54 am

both are enuf. My answer is D.

lets take 4 numbers. n, n+2, n+4, n+6

from 1, we have n+4 + n+6 = 30. Get the value of n and the numbers.

From 2, we have n+n+2 = 22. Get the value of n and the numbers.

Hence, d.

To err is human, to repeat it is criminal.

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anandsebastin: Junior | Next Rank: 30 Posts; Posts: 10; Joined: Fri Oct 13, 2006 9:16 am

by anandsebastin » Fri Oct 13, 2006 9:30 am

D

The 4 consecutive even numbers are x, x+2, x+4, x+6

Statement 1:
x+4+x+6 =30
2x=20
x+6 = 16 SUFF

Statement 2:
x+x+2 = 22
2x=20
x+6=16 SUFF

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800guy: Master | Next Rank: 500 Posts; Posts: 354; Joined: Tue Jun 27, 2006 9:20 pmFollowed by: 5 members; Thanked: 11 times

OA

by 800guy » Sun Oct 15, 2006 1:00 pm

here's the OA:

the ans is D

Soln: I guess, there are 4 consecutive even numbers to start with..
Stmt 1) Sum of 3rd even + Sum of 4th even = 30 => 14+16 = 30 => 4th even num = 16.. Sufficient

Stmt 2) Sum of Ist even +Sum of 2nd even = 22 => 10 and 12 are the 2 numbers to begin with, then 3rd num = 14 and 4th num = 16.. Sufficient

Ans – D

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countingdolls: Junior | Next Rank: 30 Posts; Posts: 13; Joined: Mon Oct 02, 2006 7:07 am

by countingdolls » Mon Oct 16, 2006 12:39 pm

Find the fourth consecutive even number:
(I) the sum of the last two numbers is 30
(II) the sum of the first two numbers is 22

If, the above is all that is given as the q, then from (I), u CANNOT find which two number in the series result in a sum of 30 (8th & 9th or 15th & the 16th)...there in no way for you to figure that out. Thats why (I) is not enuf.
In the answers posted, its assumed that the 3rd & the 4th no add up to 30.

thats why B

Pls correct me if im mistaken

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khilker: Newbie | Next Rank: 10 Posts; Posts: 4; Joined: Wed Sep 20, 2006 6:20 pm

Read again

by khilker » Tue Oct 17, 2006 8:26 am

CountingDolls: you said "u CANNOT find which two number in the series result in a sum of 30 (8th & 9th or 15th & the 16th)" but if you read the question again you should notice that:

(1) tells you that the LAST TWO numbers sum is 30.
(2) tells you that the FIRST TWO numbers sum is 22.

So you shouldn't have to worry about the 8th, 9th or other numbers. Also, the question implies that there are only four numbers in the series.

Thank you,
Ken

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countingdolls: Junior | Next Rank: 30 Posts; Posts: 13; Joined: Mon Oct 02, 2006 7:07 am

by countingdolls » Tue Oct 17, 2006 10:06 am

Hi khilker,

(1) tells you that the LAST TWO numbers sum is 30.
since we do not know how many numbers are there in the series, its not enough to find the answer.

Also, how does the question imply that there are only four numbers in the series?
Using what is given, assuming nothing, u cant conclude that tehre are just 4 nos in the series.

Isnt it not right to assume that there are just 4 numbers?

Could we discuss this because for me its out and out B?
Would some expert please comment

thanks

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rajs.kumar: Senior | Next Rank: 100 Posts; Posts: 87; Joined: Fri Jun 09, 2006 2:47 am; Thanked: 2 times

by rajs.kumar » Fri Oct 20, 2006 8:19 am

I agree with counting dolls, and I arrived at answer B.

The question stem just asks for the fourth consecutive even number. 14 and 16 could any two.

For example 2, 4, 6, 8, 10, 12, 14, 16. In this case 14 and 16 are the last two and not necessarily the fourth.

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ajith: Legendary Member; Posts: 1275; Joined: Thu Sep 21, 2006 11:13 pmFollowed by: 2 members; Location: Arabian Sea; Thanked: 125 times

by ajith » Mon Oct 23, 2006 2:57 am

rajs.kumar wrote:I agree with counting dolls, and I arrived at answer B.

The question stem just asks for the fourth consecutive even number. 14 and 16 could any two.

For example 2, 4, 6, 8, 10, 12, 14, 16. In this case 14 and 16 are the last two and not necessarily the fourth.

I agree there

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ianstrike: Community Manager; Posts: 35; Joined: Fri Sep 29, 2006 10:17 am; Location: New York, NY

by ianstrike » Mon Oct 23, 2006 8:25 am

Part of the difficulty with this problem is that it is NOT written in a GMAC way.

Find the fourth consecutive even number:
(I) the sum of the last two numbers is 30
(II) the sum of the first two numbers is 22

There is ambiguity as to whether the set is limited to four terms. Having said that, this problem never even mentioned that the terms are part of a set. GMAC doesn't do this.

Statement I is insufficient, for the previously mentioned reasons.

I think that it is important to work with actual GMAC questions (or questions that are closely modeled on GMAC style). My point is NOT that one can't learn from a problem like this one, but that the question-writer imposes ambiguities on the question in a way that is very different from what GMAC does.

Ian Streicher

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beatthegmat: Site Admin; Posts: 6778; Joined: Mon Feb 13, 2006 8:30 amFollowed by: 994 members; Location: Los Angeles, CA; Thanked: 1249 times

by beatthegmat » Mon Oct 23, 2006 9:33 am

Well said, Ian. Nothing beats working with OG as your primary practice sets, since it allows you to learn the GMAC way.

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mahesh310: Newbie | Next Rank: 10 Posts; Posts: 1; Joined: Sun Sep 04, 2011 10:22 am

by mahesh310 » Sun Sep 04, 2011 10:23 am

I- 13+17(30)
II- 9+13(22)
so the 4th number is 21

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