For a certain set of n numbers, where n > 2, is the average
(arithmetic mean) equal to the median?
(1) The n numbers are positive, consecutive even
integers.
(2) The average of the n numbers is equal to the
average of the largest and smallest numbers
in the set.
[spoiler]OA:A[/spoiler]
[spoiler]
why is statement 2 insufficient?
i tried different combinations of numbers and got mean=median:
e.g. 3,5,5,7 median=5, mean=5 mean=median . even though the set is not evenly spaced
3,4,6,7 median=5, mean=5 mean=median . also,set is not evenly spaced
i don't find a case to disprove statement 2[/spoiler]
mean median
This topic has expert replies
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 buoyant,
Your idea to TEST VALUES is perfect for this scenario. Fact 2 gives us a tricky scenario to deal with because you instinctively want to focus on positive numbers. The key to finding the "exception" that you're looking for is to think about numbers that are NOT positive and/or duplicate values.
I used (-1,-1, 3, 4, 5).
Average of biggest and smallest = (-1 + 5)/2 = 4/2 = 2
Average of all 5 terms = (12 - 2)/5 = 10/5 = 2
Median = 3
Fact 2 is INSUFFICIENT
GMAT assassins aren't born, they're made,
Rich
Your idea to TEST VALUES is perfect for this scenario. Fact 2 gives us a tricky scenario to deal with because you instinctively want to focus on positive numbers. The key to finding the "exception" that you're looking for is to think about numbers that are NOT positive and/or duplicate values.
I used (-1,-1, 3, 4, 5).
Average of biggest and smallest = (-1 + 5)/2 = 4/2 = 2
Average of all 5 terms = (12 - 2)/5 = 10/5 = 2
Median = 3
Fact 2 is INSUFFICIENT
GMAT assassins aren't born, they're made,
Rich
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
Target question: Is the average (arithmetic mean) EQUAL to the median?buoyant wrote:For a certain set of n numbers, where n > 2, is the average (arithmetic mean) equal to the median?
(1) The n numbers are positive, consecutive even integers.
(2) The average of the n numbers is equal to the average of the largest and smallest numbers in the set.
Statement 1: The n numbers are positive, consecutive even integers.
There's a nice rule that says, "In a set where the numbers are equally spaced, the mean will equal the median."
For example, in each of the following sets, the mean and median are equal:
{7, 9, 11, 13, 15}
{-1, 4, 9, 14}
{3, 4, 5, 6}
Statement 1 tells us that the numbers are consecutive even integers, which means they are equally spaced.
As such, we can be certain that the mean and median are equal.
Since we can answer the target question with certainty, statement 1 is SUFFICIENT
Statement 2: The average of the n numbers is equal to the average of the largest and smallest numbers in the set.
This statement doesn't FEEL sufficient, so I'm going to try testing some different values.
Aside: For more on this idea of testing values when a statement doesn't feel sufficient, you can read my article: https://www.beatthegmat.com/mba/2013/10/ ... -in-values
There are several different sets that satisfy this condition. Here are two:
Case a: the numbers are {1, 2, 3}. Here, the mean is equal to the average of the biggest and smallest numbers. In this case the median and mean ARE equal
Case b: the numbers are {-3, -3, 1, 2, 3}. Here, the mean is equal to the average of the biggest and smallest numbers. In this case the median and mean are NOT equal
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT
Answer = A
Cheers,
Brent
- GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Joined: Tue May 25, 2010 12:04 pm
- Location: New York, NY
- Thanked: 13060 times
- Followed by:1906 members
- GMAT Score:790
An alternate way to evaluate statement 2:
Let the 5 numbers, in ascending order, be a, b, c, d and e.
Since the average of the 5 numbers is equal to the average of a and e, we get:
(a+b+c+d+e)/5 = (a+e)/2
2(a+b+c+d+e) = 5(a+e)
2(b+c+d) + 2(a+e) = 5(a+e)
2(b+c+d) = 3(a+e)
(a+e)/(b+c+d) = 2/3.
Let a+e = 8 and b+c+d = 12, so that (a+e)/(b+c+d) = 8/12 = 2/3.
Average of the 5 numbers = (a+b+c+d+e)/5 = (8+12)/5 = 4.
Let a=0 and e=8, so that a+e = 8.
Case 1: The numbers are evenly spaced
Let b=2, c=4, and d=6, so that b+c+d = 12.
Resulting set:
0, 2, 4, 6, 8.
In this case, the average of the 5 numbers (4) is equal to the median (also 4).
Case 2: The numbers are NOT evenly spaced
Let b=3, c=3, and d=6, so that b+c+d = 3+3+6 = 12.
Resulting set:
0, 3, 3, 6, 8.
In this case, the average of the 5 numbers (4) is NOT equal to the median (3).
Thus, statement 2 is INSUFFICIENT.
Let n=5, implying a set of 5 numbers.(2) The average of the n numbers is equal to the
average of the largest and smallest numbers
in the set.
Let the 5 numbers, in ascending order, be a, b, c, d and e.
Since the average of the 5 numbers is equal to the average of a and e, we get:
(a+b+c+d+e)/5 = (a+e)/2
2(a+b+c+d+e) = 5(a+e)
2(b+c+d) + 2(a+e) = 5(a+e)
2(b+c+d) = 3(a+e)
(a+e)/(b+c+d) = 2/3.
Let a+e = 8 and b+c+d = 12, so that (a+e)/(b+c+d) = 8/12 = 2/3.
Average of the 5 numbers = (a+b+c+d+e)/5 = (8+12)/5 = 4.
Let a=0 and e=8, so that a+e = 8.
Case 1: The numbers are evenly spaced
Let b=2, c=4, and d=6, so that b+c+d = 12.
Resulting set:
0, 2, 4, 6, 8.
In this case, the average of the 5 numbers (4) is equal to the median (also 4).
Case 2: The numbers are NOT evenly spaced
Let b=3, c=3, and d=6, so that b+c+d = 3+3+6 = 12.
Resulting set:
0, 3, 3, 6, 8.
In this case, the average of the 5 numbers (4) is NOT equal to the median (3).
Thus, statement 2 is INSUFFICIENT.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3