sequence

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nicajvch: Newbie | Next Rank: 10 Posts; Posts: 9; Joined: Tue Feb 23, 2010 4:32 pm

by nicajvch » Sat May 22, 2010 3:17 pm

If the terms of a sequence ar t1, t2, t3, ..., tn what is the value of n?

1) The sum of the n ters is 3,124
2) The avg (arith mean) of the n terms is 4

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Source: Beat The GMAT — Data Sufficiency | Sat May 22, 2010 3:17 pm

liferocks: Legendary Member; Posts: 576; Joined: Sat Mar 13, 2010 8:31 pmFollowed by: 1 member; Thanked: 97 times

by liferocks » Sat May 22, 2010 9:26 pm

From 1
Sn=3124..but no relation between the terms of the series..not sufficient

From 2
Sn=4n..but Sn is not provided...not sufficient

Combining
3124=4n
or 781=n...sufficient

Ans option C

"If you don't know where you are going, any road will get you there."
Lewis Carroll

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Patrick_GMATFix: GMAT Instructor; Posts: 1052; Joined: Fri May 21, 2010 1:30 amFollowed by: 98 members; Thanked: 335 times

by Patrick_GMATFix » Sat May 22, 2010 10:42 pm

This question is easier than it seems. The little subscript numbers are indexes. They indicate the position of the number in a sequence. t1 is the first number and t3 is the 3rd number in the sequence. Because the sequence ends with tn, asking for the value of n is the same as asking for the number of terms in the sequence.

Rephrase: How many terms are in the sequence.

You have to look at the statements together since they give you 2 of the 3 parts of the average equation: avg = (Sum of terms)/(# of terms). (1) gives you the sum and (2) gives you the avg. You can solve for the # of terms. The answer is C.

To better understand, have a look at the video solution. This is GMATPrep question 1035

-Patrick

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