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How to solve?

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Senior Manager
Joined: 12 May 2010
Posts: 288
Location: United Kingdom
Concentration: Entrepreneurship, Technology
GMAT Date: 10-22-2011
GPA: 3
WE: Information Technology (Internet and New Media)
Followers: 4

Kudos [?]: 43 [0], given: 12

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13 Mar 2011, 10:10
00:00

Difficulty:

25% (medium)

Question Stats:

72% (01:00) correct 28% (01:26) wrong based on 46 sessions

HideShow timer Statictics

Six computers, each working at the same constant rate, together
can process a certain amount of data in 15 days. How many
additional computers, each working at the same constant rate,
will be needed to process the same amount of data in 10 days?
(A) 3
(B) 5
(C) 6
(D) 9
(E) 12
[Reveal] Spoiler: OA
Senior Manager
Joined: 12 May 2010
Posts: 288
Location: United Kingdom
Concentration: Entrepreneurship, Technology
GMAT Date: 10-22-2011
GPA: 3
WE: Information Technology (Internet and New Media)
Followers: 4

Kudos [?]: 43 [0], given: 12

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13 Mar 2011, 10:11
4. A
Explanation: If six computers require 15 days to process the data, thats
a total of 90 computer-days the product of 6 and 15. If you change the number
of computers or the number of days, 90 will have to remain the product, whether
that means 90 days of one computer or one day with 90 computers.
In 10 days, the number of computers is:
10c = 90
c = 9
9 computers is 3 more than the 6 that it took to do the job in 15 days, so
the correct choice is (A).

...WHAT I DON'T GET, is this business about "90 computer days"... is there another way to solve it? I wouldn't have known at all to solve it the way they solved it
Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 6477
Location: Pune, India
Followers: 1758

Kudos [?]: 10486 [1] , given: 206

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13 Mar 2011, 11:45
1
KUDOS
Expert's post
n2739178 wrote:

...WHAT I DON'T GET, is this business about "90 computer days"... is there another way to solve it? I wouldn't have known at all to solve it the way they solved it

Number of computers and days taken have an inverse relation. i.e. if number of computers increase, days taken will reduce... Agreed?

If to complete the work in 15 days, you need 6 computers,
then to complete the work in 1 day, how many computers will you need?
You will need more computers now so you multiply 15 by 6 i.e. 15*6 = 90 computers.

To complete the work in 1 day, you need 90 computers.
To complete the work in 10 days, how many computers will you need? You will need fewer computers now, right? So you divide 90 by 10 to get 90/10 = 9 computers. You need 3 extra computers.

Another way:
15 days .................. 6 computers
10 days ...................? computers
Unknown is the number of computers. You need to change the previous number of computers to get the new number. You will need to multiply either by 15/10 or 10/15. If you have fewer number of days, you will need more computers so you multiply by 15/10 (i.e. greater than 1) to give a bigger number
6*(15/10) = 9

Another way:
Days (d) and computers (c) are inversely proportional.
dc = k (a constant)
Say x is the new number of computers.
15*6 = k
10*x = k
15*6 = 10*x
x = 9

Finally, the given explanation way:
6 computers need 15 days. This means each computer is working for 15 days. How much total work is there? Work which needs 6 computers working simultaneously for 15 days so 15+15+15+15+15+15 = 90 days will be needed if only 1 computer were working.
Now, if we need to finish the total work in 10 days, how many computers do we need? 10 + 10 + 10 ..... = 90
We will need 9 such computers to finish the given work.
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Status: No dream is too large, no dreamer is too small
Joined: 14 Jul 2010
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Kudos [?]: 592 [0], given: 39

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13 Mar 2011, 12:33
To complete in 15 days 6 computers required
so '' 10 = 6*15/10 = 9
Ans A.
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Re: How to solve?   [#permalink] 13 Mar 2011, 12:33
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