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# Jeff Sackman strange explanation about simplification

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Senior Manager
Joined: 12 May 2010
Posts: 261
Location: United Kingdom
Concentration: Entrepreneurship, Technology
GMAT Date: 10-22-2011
GPA: 3
WE: Information Technology (Internet and New Media)

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22 May 2011, 07:53
Hi all, I have a question about the way this problem was solved (this is a Jeff Sackmann problem) - see the post after this one...

11. A certain playground has 9 sandboxes, 6 of which require 11 1/6 cubic feet of sand to fill, and 3 of which require 9 2/3 cubic feet of sand to fill. What is the average amount of sand needed to fill one of the 9 sandboxes?
(A) 10
(B) 10 1/6
(C) 10 5/12
(D) 10 2/3
(E) 11

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Senior Manager
Joined: 12 May 2010
Posts: 261
Location: United Kingdom
Concentration: Entrepreneurship, Technology
GMAT Date: 10-22-2011
GPA: 3
WE: Information Technology (Internet and New Media)

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22 May 2011, 07:55
Hi all the explanation is attached in the file in a screenshot.

Can someone please explain why he subtracted 9 2/3 and how that worked?

thanks
Attachments

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CEO
Status: Nothing comes easy: neither do I want.
Joined: 12 Oct 2009
Posts: 2653
Location: Malaysia
Concentration: Technology, Entrepreneurship
Schools: ISB '15 (M)
GMAT 1: 670 Q49 V31
GMAT 2: 710 Q50 V35

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22 May 2011, 09:45
You better learn the concept and solve normally.

average = {6* (67/6) + 3*(29/3) }/9 = 96/9 = 32/3 => D
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Senior Manager
Joined: 12 May 2010
Posts: 261
Location: United Kingdom
Concentration: Entrepreneurship, Technology
GMAT Date: 10-22-2011
GPA: 3
WE: Information Technology (Internet and New Media)

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22 May 2011, 13:53
thanks I know the concept but I don't know why he solved it the way he did (i.e. by subtracting 9 2/3 from each term in the numerator) ... ?
Joined: 27 Jan 2010
Posts: 141
Concentration: Strategy, Other

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22 May 2011, 14:53
n2739178 wrote:
thanks I know the concept but I don't know why he solved it the way he did (i.e. by subtracting 9 2/3 from each term in the numerator) ... ?

I am currently working through the problems in Total GMAT math and one of the methods Jeff Sackman advocates for solving weighted averages problems is to subtract the lowest number, then perform the weighted average calculation and then at the very end add back the subtracted value.

E.g: Karen purchases 3 eggs for \$17 and 7 eggs for \$23. What is the average price Karen paid for each egg?

Ans: Instead of [(3)(17) + (7)(23)]/10 = (51+161)/10 = 212/10 = 21.2

[(3)(17) + (7)(23)]/10

subtract 17 from both terms =

[(3)(0) + (7)(6)]/10 = 42/10 = 4.2

Now add back 17 = 4.2 + 17 = 21.2

The second method is much faster that calculating (3)(17) = 51 and (7)(23) = 161.

This should be used on a case to case basis.

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Re: Jeff Sackman strange explanation about simplification &nbs [#permalink] 22 May 2011, 14:53
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# Jeff Sackman strange explanation about simplification

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