Which of the following must be true of Set S?

gmattesttaker2 wrote:Hello,

Can you please assist with this:

Set S is made up of all integers n such that n is a multiple of 5 and -20 less than or equal to n less than or equal to 60, which of the following must be true of Set S?


1) If each term is multiplied by 3, its average, median and standard deviation will change.

2) The absolute value of the terms of set S will have the same standard deviation.

3) If 6 is added to each of the terms in set S, it's average (arithmetic mean) and median will change but not its standard deviation.

A) I only
B) I and II only
C) II and III only
D) III only
E) I and III only

OA: E

Thanks a lot,
Sri

Uva@90 wrote:

gmattesttaker2 wrote:Hello,

Can you please assist with this:

Set S is made up of all integers n such that n is a multiple of 5 and -20 less than or equal to n less than or equal to 60, which of the following must be true of Set S?


1) If each term is multiplied by 3, its average, median and standard deviation will change.

2) The absolute value of the terms of set S will have the same standard deviation.

3) If 6 is added to each of the terms in set S, it's average (arithmetic mean) and median will change but not its standard deviation.

A) I only
B) I and II only
C) II and III only
D) III only
E) I and III only

OA: E

Thanks a lot,
Sri

Hi Sri,
Is question stated as, n is multiple of 5 and ,-20 <= n <= 60 ?
I assume as this,
So set should be {-20,-15,-10,.........50,55,60}

As we know that,
If we add or subtract a constant to each term in a set Mean will increase or decrease by the same constant SD will not change.

So, option 3 is correct.
Eliminate A and B

Consider the second point, 'The absolute value of the terms of set S will have the same standard deviation.'

It will be true if set has all positive values,
But set has negative values too.
Hence Option 2 is Incorrect. Eliminate C

Consider Option 1.
For example consider easy numbers,
1,2,3 here SD is 1
Multiply by 3, then set is 3,6,9. Here SD is 3
Hence Option 1 is correct.

Ans is E

Regards,
Uva.

Consider Option 1.
For example consider easy numbers,
1,2,3 here SD is 1
Multiply by 3, then set is 3,6,9. Here SD is 3

Matt@VeritasPrep wrote:You and Uva are both right: 3 is the SAMPLE standard deviation, while √6 is the POPULATION standard deviation. These are different measures that are used in different situations. Here's a brief primer on the topic.

That said, you are correct (IIRC) that the GMAT uses the POPULATION standard deviation, which here would be √6, although I don't think there are any GMAT questions that depend on your making the distinction between the two.