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moneyman: Master | Next Rank: 500 Posts; Posts: 468; Joined: Sat Mar 03, 2007 10:17 pm; Thanked: 5 times

GMAT Prep

by moneyman » Sat May 10, 2008 5:56 am

List S and list T each contain 5 positive integers, and for each list the average of the integers in the list is 40. If the integers 30,40 and 50 are in both lists , is the standard deviation of the integers in list S greater than the standard deviation of the integers in list T ?

(1)The integer 25 is in list S

(2)The integer 45 is in list T

And C

What is the fastest approach ? What is the best way to calculate SD here?

Maxx

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Source: Beat The GMAT — Data Sufficiency | Sat May 10, 2008 5:56 am

Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pm; Location: Toronto; Thanked: 1710 times; Followed by:614 members; GMAT Score:800

Re: GMAT Prep

by Stuart@KaplanGMAT » Sat May 10, 2008 2:05 pm

moneyman wrote:List S and list T each contain 5 positive integers, and for each list the average of the integers in the list is 40. If the integers 30,40 and 50 are in both lists , is the standard deviation of the integers in list S greater than the standard deviation of the integers in list T ?

(1)The integer 25 is in list S

(2)The integer 45 is in list T

And C

What is the fastest approach ? What is the best way to calculate SD here?

You never need to calculate SD on the GMAT. In DS, we're just seeing if we have enough info to calculate.

In this question, if we know what the sets are, we can certainly calculate SD.

We already know 3/5 numbers on each list, so we're not far off. We also know the average of each list, and therefore the sum (= avg * # of terms).

(1) S contains 25. This allows us to determine the 5th member of S, but we don't know enough about T.

(2) T contains 45. This allows us to determine the 5th member of T, but we don't know enough about S.

Together: We know the full sets for S and T. If we know all the terms in a set, we can calculate ANYTHING about that set, including SD. If we can calculate both SDs, we can certainly answer the question: choose (c).