For a display, identical cubic boxes are stacked in square : GMAT Problem Solving (PS)
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# For a display, identical cubic boxes are stacked in square

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For a display, identical cubic boxes are stacked in square [#permalink]

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23 Sep 2013, 04:10
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For a display, identical cubic boxes are stacked in square layers. Each layer consists of cubic boxes arranged in rows that form a square, and each layer has 1 fewer row and 1 fewer box in each remaining row than the layer directly below it. If the bottom of the layer has 81 boxes and the top of the layer has only 1 box, how many boxes are in display?

A. 236
B. 260
C. 269
D. 276
E. 285
[Reveal] Spoiler: OA

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Last edited by Bunuel on 23 Sep 2013, 05:02, edited 1 time in total.
Renamed the topic and edited the question.
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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23 Sep 2013, 05:23
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imhimanshu wrote:
For a display, identical cubic boxes are stacked in square layers. Each layer consists of cubic boxes arranged in rows that form a square, and each layer has 1 fewer row and 1 fewer box in each remaining row than the layer directly below it. If the bottom of the layer has 81 boxes and the top of the layer has only 1 box, how many boxes are in display?

A. 236
B. 260
C. 269
D. 276
E. 285

Basically we have a 9-layer pyramid as shown below:
Attachment:

pyramid_with_corner_cube_from_istock.jpg [ 9.94 KiB | Viewed 24998 times ]
(Actually this pyramid 8-layer, couldn't find 9-layer one image)

The number of boxes would be: 9^2 + 8^2 + 7^2 + 6^2 + 5^2 + 4^2 + 3^2 + 2^2 + 1 = 285.

You can use the sum of the first n perfect squares formula to calculate: $$\frac{n(n+1)(2n+1)}{6}=\frac{9*(9+1)(2*9+1)}{6}=285$$.

Hope it's clear.
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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23 Sep 2013, 13:46
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Thanks bunuel but how did you manage to understand that?
I read the question again and cannot imagine the picture you uploaded in my head at all.
Thanks!
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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23 Sep 2013, 23:56
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roygush wrote:
Thanks bunuel but how did you manage to understand that?
I read the question again and cannot imagine the picture you uploaded in my head at all.
Thanks!

I read the stem carefully. We are told that:
Cubic boxes are stacked in square layers --> each layer is a square;
The bottom of the layer has 81 boxes --> the bottom layer has 9 rows and each row has 9 boxes.
Each layer has 1 fewer row and 1 fewer box in each remaining row than the layer directly below it --> the second layer has 8 rows and each row has 8 boxes.
...

Hope it helps.
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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24 Sep 2013, 07:58
1
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Bunuel wrote:
roygush wrote:
Thanks bunuel but how did you manage to understand that?
I read the question again and cannot imagine the picture you uploaded in my head at all.
Thanks!

I read the stem carefully. We are told that:
Cubic boxes are stacked in square layers --> each layer is a square;
The bottom of the layer has 81 boxes --> the bottom layer has 9 rows and each row has 9 boxes.
Each layer has 1 fewer row and 1 fewer box in each remaining row than the layer directly below it --> the second layer has 8 rows and each row has 8 boxes.
...

Hope it helps.

I got confused (and I still am) by the line which says "Each layer has 1 fewer row and 1 fewer box in each remaining row than the layer directly below it"

I got that the bottom layer will have 9 x 9 boxes
I also understand that the next level up will have 8 rows of boxes and since this layer also has to form a square hence it needs 8 boxes in the column as well.

What is elusive for me is "and 1 fewer box in each remaining row[/color] than the layer directly below it". Can you please explain again.
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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24 Sep 2013, 14:04
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violetsplash wrote:
Bunuel wrote:
roygush wrote:
Thanks bunuel but how did you manage to understand that?
I read the question again and cannot imagine the picture you uploaded in my head at all.
Thanks!

I read the stem carefully. We are told that:
Cubic boxes are stacked in square layers --> each layer is a square;
The bottom of the layer has 81 boxes --> the bottom layer has 9 rows and each row has 9 boxes.
Each layer has 1 fewer row and 1 fewer box in each remaining row than the layer directly below it --> the second layer has 8 rows and each row has 8 boxes.
...

Hope it helps.

I got confused (and I still am) by the line which says "Each layer has 1 fewer row and 1 fewer box in each remaining row than the layer directly below it"

I got that the bottom layer will have 9 x 9 boxes
I also understand that the next level up will have 8 rows of boxes and since this layer also has to form a square hence it needs 8 boxes in the column as well.

What is elusive for me is "and 1 fewer box in each remaining row[/color] than the layer directly below it". Can you please explain again.

Each layer has 1 fewer row and 1 fewer box in each remaining row than the layer directly below it:

1st layer has 9 rows and 9 boxes in each of them. 2nd row has 1 fewer, so 8 rows and each of the remaining 8 rows has 1 fewer box, so 8 boxes in it.

Hope it's clear.
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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30 Mar 2014, 15:02
Bunuel,

Do you have more examples like these questions? I am finding it difficult to understand after reading question.

Thanks

~M14
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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24 May 2014, 05:30
superb explanation Bunnel! i couldnt understand the qs...
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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28 Jul 2014, 11:55
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Easy calculation:

1) Sum the UNIT digits.
1 + 4 + 9 + 16 + 25 +36+ 49+ 64 +81

2) Agrupate in 10's
1+9
4+6
6+4
9+1
5+ ....

The unit digit must be 5, therefore the correct answer is E.
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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04 Apr 2015, 16:42
Bunuel wrote:
imhimanshu wrote:
For a display, identical cubic boxes are stacked in square layers. Each layer consists of cubic boxes arranged in rows that form a square, and each layer has 1 fewer row and 1 fewer box in each remaining row than the layer directly below it. If the bottom of the layer has 81 boxes and the top of the layer has only 1 box, how many boxes are in display?

A. 236
B. 260
C. 269
D. 276
E. 285

Basically we have a 9-layer pyramid as shown below:
Attachment:
pyramid_with_corner_cube_from_istock.jpg
(Actually this pyramid 8-layer, couldn't find 9-layer one image)

The number of boxes would be: 9^2 + 8^2 + 7^2 + 6^2 + 5^2 + 4^2 + 3^2 + 2^2 + 1 = 285.

You can use the sum of the first n perfect squares formula to calculate: $$\frac{n(n+1)(2n+1)}{6}=\frac{9*(9+1)(2*9+1)}{6}=285$$.

Hope it's clear.

Is there any chance you can apply why that 6 is there? I want to make sure I can apply this formula in more complicated cases.
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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11 Oct 2015, 18:42
Here's where non-native speakers could have trouble. By display I kept thinking of a computer display and I tried to visualize boxes arranged within the TV, and jumped into the conclusion that this was similar to a problem in the OG (13th Ed. PS 124).
Now, if you don't know the formula for the sum of the first n perfect squares (I actually forgot it on a second attempt), it is just nonsense to sum each square result. plaverbach's approach is the appropriate one. After taking a look at the answers and noticing that only two of them have the same units number, you pray that those are wrong and go ahead and find that unit.
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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03 Jan 2016, 09:17
I liked plaverbach approach, as I couldn't understand the question in the first place and choose random wrong answer.
But when I saw the picture posted, I could use the plaverbach approach.
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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29 Aug 2016, 12:01
Attachment:
File comment: Visualize the Question this Way

IMG_20160830_012335.jpg [ 1.04 MiB | Viewed 3351 times ]

In the above figures ,small circles are akin to cubic boxes and lines are rows. So bottom most layer has 9*9 = 81 boxes . Now this figure has more layers stacked on top of it and each layer has 1 less box(small circle) and 1 less row( line). If you follow this theory then you will notice that no of rows = no of boxes in each row.

So 2nd layer from the top will have 2 rows with 2 boxes each. Similarly top most layer will have 1 row and 1 box.

So the total no of boxes will be : 9* 9 + 8*8 +.....+ 1*1 = 285 .
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Re: For a display, identical cubic boxes are stacked in square [#permalink]

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20 Oct 2016, 01:00
possibly the worst worded question i've ever read
Re: For a display, identical cubic boxes are stacked in square   [#permalink] 20 Oct 2016, 01:00
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