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a, b, and c are integers and a < b < c. S is the set of all

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Director
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a, b, and c are integers and a < b < c. S is the set of all [#permalink] New post 06 Feb 2012, 03:37
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a, b, and c are integers and a < b < c. S is the set of all integers from a to b, inclusive. Q is the set of all integers from b to c, inclusive. The median of set S is (3/4) b. The median of set Q is (7/8) c. If R is the set of all integers from a to c, inclusive, what fraction of c is the median of set R?

A. 3/8
B. 1/2
C. 11/16
D. 5/7
E. 3/4

OA:
[Reveal] Spoiler:
C


Bunuel or someone else, where am I going wrong with this one?

Median of a combined interval will be in the middle between the median of Q and the median of S:

(3/4 b + 7/8 c) * 1/2 (1)

From the formula for median of Q we get:

(b+c)/2 = 7/8 c ==> b = 3/4 c (2)

Substituting b from (2) into (1) we get:

(3/4 *3/4c + 7/8 c) * 1/2 ==> 23/32 c


Please help.

Thank you.
[Reveal] Spoiler: OA
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Re: Median of a combined interval [#permalink] New post 06 Feb 2012, 03:49
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nonameee wrote:
a, b, and c are integers and a < b < c. S is the set of all integers from a to b, inclusive. Q is the set of all integers from b to c, inclusive. The median of set S is (3/4) b. The median of set Q is (7/8) c. If R is the set of all integers from a to c, inclusive, what fraction of c is the median of set R?

A. 3/8
B. 1/2
C. 11/16
D. 5/7
E. 3/4

OA:
[Reveal] Spoiler:
C


Bunuel or someone else, where am I going wrong with this one?

Median of a combined interval will be in the middle between the median of Q and the median of S:

(3/4 b + 7/8 c) * 1/2 (1)

From the formula for median of Q we get:

(b+c)/2 = 7/8 c ==> b = 3/4 c (2)

Substituting b from (2) into (1) we get:

(3/4 *3/4c + 7/8 c) * 1/2 ==> 23/32 c


Please help.

Thank you.


Given that S is the set of all integers from a to b, inclusive, Q is the set of all integers from b to c, inclusive and R is the set of all integers from a to c, inclusive, so sets S, Q and R have to be consecutive integers sets. For any set of consecutive integers (generally for any evenly spaced set) median (also the mean) equals to the average of the first and the last terms.

So we have:
Median of S=\frac{a+b}{2}=b*\frac{3}{4} --> b=2a;

Median of Q=\frac{b+c}{2}=c*\frac{7}{8} --> b=c*\frac{3}{4} --> 2a=c*\frac{3}{4} --> a=c*\frac{3}{8};

Median of R=\frac{a+c}{2}=\frac{c*\frac{3}{8}+c}{2}=c*\frac{11}{16}

Answer: C (\frac{11}{16}).
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Re: Median of a combined interval [#permalink] New post 06 Feb 2012, 03:52
Bunuel, I know the solution that you've given (I've read it in some of your previous posts).

But could you please explain where is the mistake in my solution?

Thank you.
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Re: Median of a combined interval [#permalink] New post 06 Feb 2012, 04:01
Another quick question, so for this question we're assuming that the medium is equal to mean. I thought the only way for that to happen is if there is no skewness in the set, but it doesn't say that anywhere. Is there any sort of general rule to tell if medium = mean?

Thanks so much Bunuel
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Re: Median of a combined interval [#permalink] New post 06 Feb 2012, 04:23
Can someone please explain the mistake in my original solution in the first post? Thanks a lot.
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Re: Median of a combined interval [#permalink] New post 06 Feb 2012, 04:34
nonameee wrote:
Median of a combined interval will be in the middle between the median of Q and the median of S:

(3/4 b + 7/8 c) * 1/2 (1)

From the formula for median of Q we get:

(b+c)/2 = 7/8 c ==> b = 3/4 c (2)

Substituting b from (2) into (1) we get:

(3/4 *3/4c + 7/8 c) * 1/2 ==> 23/32 c


Please help.

Thank you.


Red part is not correct: we can not assume that as we don't know that a and c are equidistant from b. If it were so then the median would simply be b.

It should be as shown in my post: (a+c)/2.
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Re: Median of a combined interval [#permalink] New post 06 Feb 2012, 04:39
kys123 wrote:
Another quick question, so for this question we're assuming that the medium is equal to mean. I thought the only way for that to happen is if there is no skewness in the set, but it doesn't say that anywhere. Is there any sort of general rule to tell if medium = mean?

Thanks so much Bunuel


For any evenly spaced set (aka AP) the arithmetic mean (average) is equal to the median (consecutive integers are evenly spaced set).
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COLLECTION OF QUESTIONS:
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Re: Median of a combined interval [#permalink] New post 06 Feb 2012, 05:11
Bunuel, so in order to determine a median of two intervals of integers (a,b) and (b,c) (where a<b<c), you should always use the formula: (a+c)/2?
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Re: Median of a combined interval [#permalink] New post 06 Feb 2012, 05:20
nonameee wrote:
Bunuel, so in order to determine a median of two intervals of integers (a,b) and (b,c) (where a<b<c), you should always use the formula: (a+c)/2?


The median (mean) of the integers from a to c, inclusive is always (a+c)/2 (if you have some additional info you can obtain this value in another way but this way is ALWAYS true).

Consider two sets: {1, 2, 3} and {3, 4, 5, 6, 7, 8, 9} --> combined set {1, 2, 3, 4, 5, 6, 7 8, 9}

As you've written the median (mean) of combined set should be (2+6)/2=4, which is wrong as median of combined set is 5.

Hope it's clear.
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Re: Median of a combined interval [#permalink] New post 06 Feb 2012, 05:22
Yes, thanks a lot. I got it.
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Re: Median of a combined interval [#permalink] New post 03 Dec 2012, 11:14
Bunuel wrote:
kys123 wrote:
Another quick question, so for this question we're assuming that the medium is equal to mean. I thought the only way for that to happen is if there is no skewness in the set, but it doesn't say that anywhere. Is there any sort of general rule to tell if medium = mean?

Thanks so much Bunuel


For any evenly spaced set (aka AP) the arithmetic mean (average) is equal to the median (consecutive integers are evenly spaced set).



Bunuel,

how do we know that they are evenly spaced. The a<b< c can be 1<2<3 or random 4<78<125 (not evenly spaced). Am i missing something?
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Re: Median of a combined interval [#permalink] New post 04 Dec 2012, 04:15
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aditi2013 wrote:
Bunuel wrote:
kys123 wrote:
Another quick question, so for this question we're assuming that the medium is equal to mean. I thought the only way for that to happen is if there is no skewness in the set, but it doesn't say that anywhere. Is there any sort of general rule to tell if medium = mean?

Thanks so much Bunuel


For any evenly spaced set (aka AP) the arithmetic mean (average) is equal to the median (consecutive integers are evenly spaced set).



Bunuel,

how do we know that they are evenly spaced. The a<b< c can be 1<2<3 or random 4<78<125 (not evenly spaced). Am i missing something?


Given that "S is the set of all integers from a to b, inclusive" and "Q is the set of all integers from b to c, inclusive", which means that both S and Q are sets of consecutive integers, thus evenly spaced sets.

Hope it's clear.
_________________

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. NEW!!! ,11 Mixed Questions NEW!!!, 12 Fresh Meat NEW!!!

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set. NEW!!!, 11 New DS set. NEW!!!


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Re: Median of a combined interval   [#permalink] 04 Dec 2012, 04:15
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