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Jean puts N identical cubes, the sides of which are 1 inch [#permalink]
03 Sep 2012, 01:16

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Difficulty:

85% (hard)

Question Stats:

46% (02:41) correct
54% (01:28) wrong based on 129 sessions

Jean puts N identical cubes, the sides of which are 1 inch long, inside a rectangular box, each side of which is longer than 1 inch, such that the box is completely filled with no gaps and no cubes left over. What is N?

In the explanation of this problem I can't figure out why " To be able to put N cubes into a rectangular box with no gaps and no left-over cubes, you must have the following equality: N = length × width × height. Moreover, if the length, width, and height are all greater than 1, then it must be true that N is the product of at least 3 primes. If N is itself prime or the product of just 2 primes (unique or not), then the condition fails.

So you can rephrase the question this way: is N the product of at least 3 primes?"

Can you help me ?? the key of this problem is just this process of thought.......

Re: Jean puts N identical cubes, the sides of which are 1 inch [#permalink]
03 Sep 2012, 01:44

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Expert's post

Jean puts N identical cubes, the sides of which are 1 inch long, inside a rectangular box, each side of which is longer than 1 inch, such that the box is completely filled with no gaps and no cubes left over. What is N?

Notice that, since the volume of each cube is 1 inch^3, then the volume of N cubes (the volume of the rectangular box) is N inch^3. For example if there are 10 cubes, then the volume of the rectangular box (total volume of 10 cubes) is 10 inch^3. Next, we are told that the length, the width and the height of the rectangular box is longer than 1 inch and there are no gaps when all cubes are put in the box, so the length, the width and the height of the rectangular box are integers more than one: 2, 3, 4, ... Thus each dimension of the rectangular box must have at least one prime in it, so the volume (length*width*height) must be the product of at least 3 primes (not necessarily distinct primes).

(1) 56 < N < 63. N could be 57, 58, 59, 60, 61, or 62. Analyze each case:

57=3*19 --> just two primes. Discard. 58=2*29 --> just two primes. Discard. 59 --> prime itself. Discard. 60=2^2*3*5 --> the product of 4 primes. OK. For example, the the length, the width and the height of the cube cold be 2, by 6, by 5. 61 --> prime itself. Discard. 62=2*31 --> just two primes. Discard.

As we can see, N can only be 60. Sufficient.

(2) N is a multiple of 3. Multiple values of N are possible so that it to be a multiple of 3 AND the product of at least 3 primes, for example 27 or 60. Not sufficient.

Re: Jean puts N identical cubes, the sides of which are 1 inch [#permalink]
06 Sep 2012, 07:47

Bunuel wrote:

Jean puts N identical cubes, the sides of which are 1 inch long, inside a rectangular box, each side of which is longer than 1 inch, such that the box is completely filled with no gaps and no cubes left over. What is N?

Notice that, since the volume of each cube is 1 inch^3, then the volume of N cubes (the volume of the rectangular box) is N inch^3. For example if there are 10 cubes, then the volume of the rectangular box (total volume of 10 cubes) is 10 inch^3. Next, we are told that the length, the width and the height of the rectangular box is longer than 1 inch and there are no gaps when all cubes are put in the box, so the length, the width and the height of the rectangular box are integers more than one: 2, 3, 4, ... Thus each dimension of the rectangular box must have at least one prime in it, so the volume (length*width*height) must be the product of at least 3 primes (not necessarily distinct primes).

(1) 56 < N < 63. N could be 57, 58, 59, 60, 61, or 62. Analyze each case:

57=3*19 --> just two primes. Discard. 58=2*29 --> just two primes. Discard. 59 --> prime itself. Discard. 60=2^2*3*5 --> the product of 4 primes. OK. For example, the the length, the width and the height of the cube cold be 2, by 6, by 5. 61 --> prime itself. Discard. 62=2*31 --> just two primes. Discard.

As we can see, N can only be 60. Sufficient.

(2) N is a multiple of 3. Multiple values of N are possible so that it to be a multiple of 3 AND the product of at least 3 primes, for example 27 or 60. Not sufficient.

Answer: A.

Hope it's clear.

Hi Bunnel,

The question says "each side of which is longer than 1 inch" so the lengths of each sides can even be "2, 2, 2, or 3,3,3" may i know why you took the lenghts as "2,3,4"

Re: Jean puts N identical cubes, the sides of which are 1 inch [#permalink]
06 Sep 2012, 09:06

1

This post received KUDOS

Expert's post

kotela wrote:

Bunuel wrote:

Jean puts N identical cubes, the sides of which are 1 inch long, inside a rectangular box, each side of which is longer than 1 inch, such that the box is completely filled with no gaps and no cubes left over. What is N?

Notice that, since the volume of each cube is 1 inch^3, then the volume of N cubes (the volume of the rectangular box) is N inch^3. For example if there are 10 cubes, then the volume of the rectangular box (total volume of 10 cubes) is 10 inch^3. Next, we are told that the length, the width and the height of the rectangular box is longer than 1 inch and there are no gaps when all cubes are put in the box, so the length, the width and the height of the rectangular box are integers more than one: 2, 3, 4, ... Thus each dimension of the rectangular box must have at least one prime in it, so the volume (length*width*height) must be the product of at least 3 primes (not necessarily distinct primes).

(1) 56 < N < 63. N could be 57, 58, 59, 60, 61, or 62. Analyze each case:

57=3*19 --> just two primes. Discard. 58=2*29 --> just two primes. Discard. 59 --> prime itself. Discard. 60=2^2*3*5 --> the product of 4 primes. OK. For example, the the length, the width and the height of the cube cold be 2, by 6, by 5. 61 --> prime itself. Discard. 62=2*31 --> just two primes. Discard.

As we can see, N can only be 60. Sufficient.

(2) N is a multiple of 3. Multiple values of N are possible so that it to be a multiple of 3 AND the product of at least 3 primes, for example 27 or 60. Not sufficient.

Answer: A.

Hope it's clear.

Hi Bunnel,

The question says "each side of which is longer than 1 inch" so the lengths of each sides can even be "2, 2, 2, or 3,3,3" may i know why you took the lenghts as "2,3,4"

Regards Srinath

I said that "the length, the width and the height of the rectangular box are integers more than one: 2, 3, 4, ... " So, yes, each dimension can be 2 or 3, for example . _________________

Re: Jean puts N identical cubes, the sides of which are 1 inch [#permalink]
06 Sep 2012, 17:37

Bunuel wrote:

kotela wrote:

Bunuel wrote:

Jean puts N identical cubes, the sides of which are 1 inch long, inside a rectangular box, each side of which is longer than 1 inch, such that the box is completely filled with no gaps and no cubes left over. What is N?

Notice that, since the volume of each cube is 1 inch^3, then the volume of N cubes (the volume of the rectangular box) is N inch^3. For example if there are 10 cubes, then the volume of the rectangular box (total volume of 10 cubes) is 10 inch^3. Next, we are told that the length, the width and the height of the rectangular box is longer than 1 inch and there are no gaps when all cubes are put in the box, so the length, the width and the height of the rectangular box are integers more than one: 2, 3, 4, ... Thus each dimension of the rectangular box must have at least one prime in it, so the volume (length*width*height) must be the product of at least 3 primes (not necessarily distinct primes).

(1) 56 < N < 63. N could be 57, 58, 59, 60, 61, or 62. Analyze each case:

57=3*19 --> just two primes. Discard. 58=2*29 --> just two primes. Discard. 59 --> prime itself. Discard. 60=2^2*3*5 --> the product of 4 primes. OK. For example, the the length, the width and the height of the cube cold be 2, by 6, by 5. 61 --> prime itself. Discard. 62=2*31 --> just two primes. Discard.

As we can see, N can only be 60. Sufficient.

(2) N is a multiple of 3. Multiple values of N are possible so that it to be a multiple of 3 AND the product of at least 3 primes, for example 27 or 60. Not sufficient.

Answer: A.

Hope it's clear.

Hi Bunnel,

The question says "each side of which is longer than 1 inch" so the lengths of each sides can even be "2, 2, 2, or 3,3,3" may i know why you took the lenghts as "2,3,4"

Regards Srinath

I said that "the length, the width and the height of the rectangular box are integers more than one: 2, 3, 4, ... " So, yes, each dimension can be 2 or 3, for example .

Re: Jean puts N identical cubes, the sides of which are 1 inch [#permalink]
09 Aug 2014, 12:06

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