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The average (arithmetic mean) age of the buildings on a cert

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The average (arithmetic mean) age of the buildings on a cert [#permalink] New post 30 Aug 2018, 07:46
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Question Stats:

45% (01:32) correct 54% (01:50) wrong based on 11 sessions
The average (arithmetic mean) age of the buildings on a certain city block is greater than 40 years old. If four of the buildings were built two years ago and none of the buildings are more than 80 years old, which of the following could be the number of buildings on the block?

Indicate all such numbers.

A. 4
B. 6
C. 8
D. 11
E. 40
[Reveal] Spoiler: OA

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Re: The average (arithmetic mean) age of the buildings on a cert [#permalink] New post 31 Aug 2018, 21:03
How to solve it?
please explain
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Re: The average (arithmetic mean) age of the buildings on a cert [#permalink] New post 12 Sep 2018, 00:50
explanation pls.??
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Re: The average (arithmetic mean) age of the buildings on a cert [#permalink] New post 12 Sep 2018, 07:59
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\(40x= (x-4)(80)\)

\(-40x=-320\)

\(x= 7.8\)

x must be at least 8 to bring the average age up over 40.

Hope this helps.

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Re: The average (arithmetic mean) age of the buildings on a cert [#permalink] New post 11 Jun 2019, 07:26
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if (x-4)80=40x, then (x-4)80 keeps average equal to 40 and there are the rest 4 buildings aged 2 years less than others .
Therefore, the value X ( integer as it is building number )decides the range of the number of buildings.
Thank you,
It helps immensely .
regards
Re: The average (arithmetic mean) age of the buildings on a cert   [#permalink] 11 Jun 2019, 07:26
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The average (arithmetic mean) age of the buildings on a cert

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