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Each of the 30 boxes in a certain shipment weighs either 10

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Each of the 30 boxes in a certain shipment weighs either 10 [#permalink] New post 09 Oct 2009, 22:25
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Each of the 30 boxes in a certain shipment weighs either 10 pounds or 20 pounds, and average (arithmetic mean) weight of the boxes in the shipment is 18 pounds. If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes, how many 20-pound boxes must be removed?

A. 4
B. 6
C. 10
D. 20
E. 24

[Reveal] Spoiler:
OA D IMO B
[Reveal] Spoiler: OA
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Re: Boxes [#permalink] New post 09 Oct 2009, 22:43
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reply2spg wrote:
Each of the 30 boxes in a certain shipment weighs either 10 pounds or 20 pounds, and average (arithmetic mean) weight of the boxes in the shipment is 18 pounds. If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes, how many 20-pound boxes must be removed?

A. 4
B. 6
C. 10
D. 20
E. 24

[Reveal] Spoiler:
OA D IMO B


x = 20 lb box:

20x + 10(30-x) = 18(30)
x = (540 - 300)/10
x = 24

If k = removed 20 lb box:

20(24-k)+ 10(6) = 14(6+24-k)
480 - 20k + 60 = 420 - 14k
540 - 420 = 6k
540 - 420 = 6k
k = 120/6 = 20
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Re: Boxes [#permalink] New post 09 Oct 2009, 23:07
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reply2spg wrote:
Each of the 30 boxes in a certain shipment weighs either 10 pounds or 20 pounds, and average (arithmetic mean) weight of the boxes in the shipment is 18 pounds. If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes, how many 20-pound boxes must be removed?

A. 4
B. 6
C. 10
D. 20
E. 24

[Reveal] Spoiler:
OA D IMO B


This is a weighted average question: Weighted \ average=\frac{weight_1*value_1+weight_2*value_2}{value_1+value_2}.

Let x be number of 10-pound boxes --> \frac{10x + 20*(30-x)}{30}=18 --> x=6;

We have 6 10-pound boxes and 24 20-pound boxes.

Let y be number of 20-pound boxes that should be removed so that average to become 14 pounds --> \frac{10*6+20*(24-y)}{30-y}=14 --> y=20.

Answer: D.
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Re: Boxes [#permalink] New post 12 Dec 2010, 23:28
dont you have to reduce the average weight of 30 booxes to 14 pounds?
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Re: Boxes [#permalink] New post 13 Dec 2010, 01:47
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aalriy wrote:
dont you have to reduce the average weight of 30 booxes to 14 pounds?


Yes, the average weight of the 30 boxes is to be reduced to 14 pounds by removing some of the 20-pound boxes.

We found that there were 6 10-pound boxes and 24 20-pound boxes.

So, if y is the number of 20-pound boxes that should be removed so that average to become 14 pounds --> \frac{10*6+20*(24-y)}{30-y}=14, (note that as we are reducing the # of 20-pound boxes (24) by y then the total # of boxes (30) also will be reduced by y) --> y=20.

Hope it's clear.
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Re: Boxes [#permalink] New post 13 Dec 2010, 03:43
Bunuel wrote:
So, if y is the number of 20-pound boxes that should be removed so that average to become 14 pounds --> \frac{10*6+20*(24-y)}{30-y}=14, (note that as we are reducing the # of 20-pound boxes (24) by y then the total # of boxes (30) also will be reduced by y--> is this implied or an assumption) --> y=20.

.


See my comment in Red Italics. What if we make an attempt to keep total number of boxes 30 by adding y 10-pound boxes. Does this have to be specifically mentioned in the question?
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Re: Boxes [#permalink] New post 13 Dec 2010, 04:15
x boxes of 10pounds
y boxes of 20pounds
x+y = 30

(10*x+20*y)/30=18

10x+20y=540

Let ? be the number of 20 pound boxes which should be removed in order to get the average weight of 14pounds

(10*x+20*y-20*?)/(30-?)=14

(540-20*?)= (420-14*?)

? = (120/6) = 20
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Re: Boxes [#permalink] New post 13 Dec 2010, 04:53
Expert's post
aalriy wrote:
Bunuel wrote:
So, if y is the number of 20-pound boxes that should be removed so that average to become 14 pounds --> \frac{10*6+20*(24-y)}{30-y}=14, (note that as we are reducing the # of 20-pound boxes (24) by y then the total # of boxes (30) also will be reduced by y--> is this implied or an assumption) --> y=20.

.


See my comment in Red Italics. What if we make an attempt to keep total number of boxes 30 by adding y 10-pound boxes. Does this have to be specifically mentioned in the question?


You should read the stem more carefully: "If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes, how many 20-pound boxes must be removed?" So stem mentions that the reduction in average weight should be made by removing some of the 20-pound boxes.

Hope it's clear.
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Re: Boxes [#permalink] New post 13 Dec 2010, 21:02
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reply2spg wrote:
Each of the 30 boxes in a certain shipment weighs either 10 pounds or 20 pounds, and average (arithmetic mean) weight of the boxes in the shipment is 18 pounds. If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes, how many 20-pound boxes must be removed?

A. 4
B. 6
C. 10
D. 20
E. 24

[Reveal] Spoiler:
OA D IMO B


Long question stem. Best approach here would be to analyze one sentence at a time.

Each of the 30 boxes in a certain shipment weighs either 10 pounds or 20 pounds, and average (arithmetic mean) weight of the boxes in the shipment is 18 pounds.

If average of 10 and 20 is 18, the ratio of no. of 10 pound boxes: no of 20 pound boxes is 1:4
(If this concept is not clear, check out http://gmatclub.com/forum/tough-ds-105651.html#p828579)
So out of 30 boxes, 6 are 10 pound boxes and 24 are 20 pound boxes.

If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes

Now average of 10 and 20 pound boxes is to be 14. So ratio of no. of 10 pound boxes:no. of 20 pound boxes = 3:2
The number of 10 pound boxes remain the same so we still have 6 of them. To get a ratio of 3:2, the no of 20 pound boxes must be 4.

how many 20-pound boxes must be removed?
There were 24 of these. Now there are only 4. 24 - 4 = 20 should be removed.
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Re: Boxes [#permalink] New post 21 Aug 2011, 09:59
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total current weight = 30*18 = 540
let's assume that x number of 20lbs boxes will be removed

\frac{540-20x}{30-x} = 14

x = 20 ............. D
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Re: Boxes [#permalink] New post 10 Feb 2012, 05:35
N1+ N2 = 30

10(N1) + 20(N1)
_____________ =18
N1+ N2

----- > solve 10(N1) + 20(N2) = 18(N1) + 18(N2)

So n1 =6 n2 = 24

Now N1 will not change ONLY n2 will change so

reduce the average to 14

10(6) + 20(N2) = 14(6) + 14(N2)

N2 =4

So it has to reduce by 20
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Re: Each of the 30 boxes in a certain shipment weighs either 10 [#permalink] New post 29 May 2013, 04:13
This is how I solved

X+Y =30

from condition given

10X+20Y = 540

therefore X=6 and Y = 24

Condition 2
X= 6 Y changes by "n" and avg weight of 30-n =14

therefore

10*6 + 20(24-n)=14( 30-n)

after solving the above equation we have

n=20

Ans is D
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Re: Boxes [#permalink] New post 31 May 2013, 12:55
MBAhereIcome wrote:
total current weight = 30*18 = 540
let's assume that x number of 20lbs boxes will be removed

\frac{540-20x}{30-x} = 14

x = 20 ............. D


Faster to use this logic
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Re: Each of the 30 boxes in a certain shipment weighs either 10 [#permalink] New post 10 Jun 2013, 08:24
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reply2spg wrote:
Each of the 30 boxes in a certain shipment weighs either 10 pounds or 20 pounds, and average (arithmetic mean) weight of the boxes in the shipment is 18 pounds. If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes, how many 20-pound boxes must be removed?

A. 4
B. 6
C. 10
D. 20
E. 24

[Reveal] Spoiler:
OA D IMO B


Easy question:

suppose 10 pound boxes = x
20 pound boxes = (30-x)

Given in question >> [x*10+(30-x)*20]/30=18

=> [x*10+(30-x)*20] = 540 ----eq. 1 .. Leave it like this.

Now we have to remove some 20 Pound boxes

let 20 pound boxes removed = a
new equation will be >> [x*10+(30-x-a)*20]/(30-a)=14
>> x*10+(30-x)*20 - 20a = 14(30-a)
From eq 1 >> 540 - 20a = 420 -14a
solve for a, and you'll get a=20 ..

I made it look quite big, but I solved it under 2 minutes using this in first try on such question
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Re: Each of the 30 boxes in a certain shipment weighs either 10 [#permalink] New post 16 Aug 2014, 11:07
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Re: Each of the 30 boxes in a certain shipment weighs either 10   [#permalink] 16 Aug 2014, 11:07
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