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# official guide (OG) 11th edition 215

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official guide (OG) 11th edition 215 [#permalink]

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16 Apr 2009, 07:35
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if m is the average (arithmetic mean) of the first 10 positive multiples of 5 and if M is the median of the first 10 positive multiples of 5, what is the value of M-m?

a. -5
b. 0
c. 5
d. 25
e. 27.5

according to the answer in OG, the first 10 positive multiples are 1,2,3, ...,8,9,10, but i think positive number could also be 1.1, 1.2 .... or some other number which bigger than 0.

the correct answer is e, but why? Thks a lot
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Re: official guide (OG) 11th edition 215 [#permalink]

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16 Apr 2009, 08:22
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KUDOS
"MULTIPLE" by definition has to be resulted when multiplied by a an Integer.
For example, 10 is a multiple of 5 because when 5 is multiplied by 2, it results in 10.
In the same way, 5 x 1.5 = 7.5 doesn't imply that 7.5 is a multiple of 5 because 1.5 is not an integer.

First 10 positive multiples of 5 are = 5,10,15,20,25,30,35,40,45,50.
Arithmetic mean would be = 5(1+2+3......9+10)/10 = 27.5

In the same way, Median = (25+30)/2 = 27.5

Thus, M-m = 0.

Official Answer as listed in OG 11 is B.
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Re: official guide (OG) 11th edition 215 [#permalink]

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20 Apr 2009, 06:57
Goldenagle94,

thks a lot:)
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Re: official guide (OG) 11th edition 215 [#permalink]

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20 Apr 2009, 08:42
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Here one important rule to remember is that, for equally spaced numbers median=mean. Therefore, mean-median=0. we dont need to solve it.
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Re: official guide (OG) 11th edition 215 [#permalink]

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21 Apr 2009, 06:17
bandit wrote:
Here one important rule to remember is that, for equally spaced numbers median=mean. Therefore, mean-median=0. we dont need to solve it.

thks.
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Re: official guide (OG) 11th edition 215 [#permalink]

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21 Apr 2009, 06:45
bandit wrote:
Here one important rule to remember is that, for equally spaced numbers median=mean. Therefore, mean-median=0. we dont need to solve it.

what does "equally spaced" mean?
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Re: official guide (OG) 11th edition 215 [#permalink]

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21 Apr 2009, 23:52
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kbulse wrote:
bandit wrote:
Here one important rule to remember is that, for equally spaced numbers median=mean. Therefore, mean-median=0. we dont need to solve it.

what does "equally spaced" mean?

For numbers in arithmatic progression (AP). For example

1,2,3,4,5,6
5,10,15,20,25
3,6,9,12
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Re: official guide (OG) 11th edition 215 [#permalink]

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23 Apr 2009, 13:40
smart tip by goldeneagle and bandit
Re: official guide (OG) 11th edition 215   [#permalink] 23 Apr 2009, 13:40
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# official guide (OG) 11th edition 215

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