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## GMAT PREP Q

This topic has 3 member replies
yvonne12 Master | Next Rank: 500 Posts
Joined
07 Feb 2007
Posted:
144 messages

#### GMAT PREP Q

Wed Mar 07, 2007 3:16 pm
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])
A CERTAIN LIST OF 100 DATA HAS AN AVERAGE OF 6 AND A STANDARD DEVIATION OF D WHERE D IS POSITIVE . WHICH OF THE FOLLOWING PAIRS OF DATE, WHEN ADED TO THE LIST , MUST RESULT IN A LIST OF 102 DATA W STANDARD DEVIATION LESS THAN D?
-6 AND 0
0 AND 0
0 AND 6
0 AND 12

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ssiva Junior | Next Rank: 30 Posts
Joined
13 Feb 2007
Posted:
16 messages
Thanked:
1 times
Wed Mar 07, 2007 4:25 pm
I am not sure but is it 0 and 0 ?
as sum( sqaures of ( x-x' ) ) for 0 and 0 is the lowest.

jayhawk2001 Community Manager
Joined
28 Jan 2007
Posted:
789 messages
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Wed Mar 07, 2007 7:43 pm
Adding 2 numbers to the list actually changes the average unless both
numbers are 6. We don't have an option (6 and 6) but lets assume that
the average is in the vicinity of 6 (atleast looking at the choices).

Since SD = sum (square (x - avg)) / N, we need to choose the numbers
that have the least value for square (x - avg).

If we consider the average as unchanged, the numbers that yield the
least sum = 0 and 6. This would cause the numerator to increase by
just 36.

-6 and 0 increases sum by 140
0 and 0 increases sum by 72
0 and 6 increases sum by 36
0 and 12 increases sum by 72

yvonne12 Master | Next Rank: 500 Posts
Joined
07 Feb 2007
Posted:
144 messages
Wed Mar 07, 2007 9:15 pm
You're right. I accidentally left out 6 and 6. JAyhawk...you're so smart

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