What is SD of given set of numbers whose average is 5?
(1) None of the numbers are greater than this Average
(2) The Standard deviation is 5 when value of each of the given number is increased by 7
OA IS D
What is SD of given set of numbers whose average is 5?
This topic has expert replies
- sachin_yadav
- Master | Next Rank: 500 Posts
- Posts: 212
- Joined: Mon Dec 06, 2010 12:52 am
- Location: India
- Thanked: 5 times
- Followed by:1 members
GMAT/MBA Expert
- [email protected]
- Elite Legendary Member
- Posts: 10392
- Joined: Sun Jun 23, 2013 6:38 pm
- Location: Palo Alto, CA
- Thanked: 2867 times
- Followed by:511 members
- GMAT Score:800
Hi sachin_yadav,
What is the source of this question? I ask because the 'design' of this question is incorrect (the two Facts CANNOT BOTH be sufficient while giving different answers).
We're told that the average of a group of numbers is 5. We're asked for the Standard Deviation of the group.
1) None of the numbers are greater than this Average
The ONLY way that this can occur is if ALL of the numbers in the set are the SAME number. Thus, while we don't know how many numbers are in the set, we do know that they are all 5s. By extension, the SD = 0
Fact 1 is SUFFICIENT.
2) The Standard deviation is 5 when value of each of the given number is increased by 7
This Fact doesn't tell us what the numbers are, BUT if we increase EACH number by 7, then the Stand Deviation would still the SAME. By extension, the SD = 5
Fact 2 is SUFFICIENT.
This would make the answer D, but the GMAT does NOT create its DS questions in this way. The SD CANNOT be 0 and 5.
GMAT assassins aren't born, they're made,
Rich
What is the source of this question? I ask because the 'design' of this question is incorrect (the two Facts CANNOT BOTH be sufficient while giving different answers).
We're told that the average of a group of numbers is 5. We're asked for the Standard Deviation of the group.
1) None of the numbers are greater than this Average
The ONLY way that this can occur is if ALL of the numbers in the set are the SAME number. Thus, while we don't know how many numbers are in the set, we do know that they are all 5s. By extension, the SD = 0
Fact 1 is SUFFICIENT.
2) The Standard deviation is 5 when value of each of the given number is increased by 7
This Fact doesn't tell us what the numbers are, BUT if we increase EACH number by 7, then the Stand Deviation would still the SAME. By extension, the SD = 5
Fact 2 is SUFFICIENT.
This would make the answer D, but the GMAT does NOT create its DS questions in this way. The SD CANNOT be 0 and 5.
GMAT assassins aren't born, they're made,
Rich
- sachin_yadav
- Master | Next Rank: 500 Posts
- Posts: 212
- Joined: Mon Dec 06, 2010 12:52 am
- Location: India
- Thanked: 5 times
- Followed by:1 members
Hi Rich,
Thanks for your reply. I got this question from GMAT club while i was practising. I totally agree that both statements are contradicting each other in answers. However, it's a good question because it tests on concepts.
I am a bit weak in Standard deviation concepts.
Regards
Sachin
Thanks for your reply. I got this question from GMAT club while i was practising. I totally agree that both statements are contradicting each other in answers. However, it's a good question because it tests on concepts.
I am a bit weak in Standard deviation concepts.
Regards
Sachin
Never surrender
GMAT/MBA Expert
- Brent@GMATPrepNow
- GMAT Instructor
- Posts: 16207
- Joined: Mon Dec 08, 2008 6:26 pm
- Location: Vancouver, BC
- Thanked: 5254 times
- Followed by:1268 members
- GMAT Score:770
It's important to note that, for the purposes of the GMAT, we need not involve the actual standard deviation. Instead, it's sufficient to think of Standard Deviation as the Average Distance from the Mean. This is a convenient way to handle most standard deviation questions on the GMAT.sachin_yadav wrote: I am a bit weak in Standard deviation concepts.
Regards
Sachin
Here are two free videos that cover everything you need to know about standard deviation:
- https://www.gmatprepnow.com/module/gmat- ... ics?id=806
- https://www.gmatprepnow.com/module/gmat- ... ics?id=809
Here are a few more practice questions to try:
- https://www.beatthegmat.com/standard-dev ... 74384.html
- https://www.beatthegmat.com/standard-dev ... 69584.html
- https://www.beatthegmat.com/range-and-sd-t89159.html
Cheers,
Brent