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If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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Question Stats: 84% (01:18) correct 16% (01:44) wrong based on 1465 sessions

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If k is a positive integer, what is the remainder when (k + 2)(k^3 – k) is divided by 6 ?

A. 0
B. 1
C. 2
D. 3
E. 4

Originally posted by AbdurRakib on 17 Jun 2017, 08:32.
Last edited by Bunuel on 17 Jun 2017, 12:18, edited 1 time in total.
Renamed the topic and edited the question.
Math Expert V
Joined: 02 Sep 2009
Posts: 59712
Re: If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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25
AbdurRakib wrote:
If k is a positive integer, what is the remainder when (k + 2)(k^3 – k) is divided by 6 ?

A. 0
B. 1
C. 2
D. 3
E. 4

There is a rule saying that: The product of n consecutive integers is always divisible by n!.

$$(k + 2)(k^3 – k) = (k + 2)k(k^2 – 1) = (k + 2)k(k – 1)(k+1)=(k – 1)k(k+1)(k + 2)$$.

As we can see we have the product of 4 consecutive integers: (k – 1), k, (k+1), and (k + 2). Thus, the product will always be divisible by 4! = 24, which means that it will be divisible by 6 too.

Of course we can simply plug-in numbers: since PS question cannot have more than 1 correct answer, then for any k the answer must be the same. For example, if k = 1, then the product becomes 0. 0 divided by 6 gives the remainder of 0 (0 is a multiple of every integer, except 0 itself).
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Re: If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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11
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AbdurRakib wrote:
If k is a positive integer, what is the remainder when (k + 2)($$k^{3}$$ – k) is divided by 6 ?

A. 0
B. 1
C. 2
D. 3
E. 4

$$(k + 2)(k^{3}$$ – k) $$= (k + 2) (k (k^2-1))$$

$$(k + 2) k(k+1)(k-1$$)

or could be written as

$$(k-1)k(k+1)(k+2)$$

Its product of 4 consecutive numbers. Product of 3 consecutive numbers is always divisible by 6, Hence given number is divisible by 6.

Therefore remainder will be 0. Answer (A).
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Re: If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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0. As k can be shown as product of three integers. Always divisible by 6

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Re: If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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BeingHan wrote:
0. As k can be shown as product of three integers. Always divisible by 6

Did you mean that k is a product of three *consecutive* integers? Then you are correct, always divisible by 6.
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Re: If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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8
$$(k + 2) (k^3 – k)$$

$$(k + 2) (k (K^2 - 1))$$

$$(k + 2) k (k + 1) (k - 1)$$

$$(k - 1) k (k + 1) (k + 2)$$

As we can see that above numbers are 4 consecutive integers, and as we know $$4$$ consecutive integers are multiples of $$4! = 4 * 3 * 2 * 1 = 24$$

Hence, as $$24$$is divisible by $$6$$, equation will be commpletely divisible by $$6$$ and hence reminder will be ZERO.

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If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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AbdurRakib wrote:
If k is a positive integer, what is the remainder when (k + 2)(k³ – k) is divided by 6 ?

A. 0
B. 1
C. 2
D. 3
E. 4

Alternatively, we can plug in any positive integer for k and see what happens.

Try k = 1
We get: (k + 2)(k³ – k) = (1 + 2)(1³ – 1)
= (3)(0)
= 0
When we divide 0 by 6 we get 0 with remainder 0

Just for "fun," let's test another k-value
Try k = 2
We get: (k + 2)(k³ – k) = (2 + 2)(2³ – 2)
= (4)(6)
= 24
When we divide 24 by 6 we get 4 with remainder 0

Try k = 3
We get: (k + 2)(k³ – k) = (3 + 2)(3³ – 3)
= (5)(24)
= 120
When we divide 120 by 6 we get 20 with remainder 0

And so on...

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Originally posted by GMATPrepNow on 28 Jun 2017, 06:58.
Last edited by GMATPrepNow on 13 Nov 2018, 09:21, edited 1 time in total.
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If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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AbdurRakib wrote:
If k is a positive integer, what is the remainder when (k + 2)(k^3 – k) is divided by 6 ?

A. 0
B. 1
C. 2
D. 3
E. 4

$$(k + 2)(k^3 – k)$$
= $$(k + 2)k(k^2 – 1)$$
= $$(k + 2)k(k – 1)(k + 1)$$

Plug in numbers and check -

Let k = 2

$$(k + 2)k(k – 1)(k + 1)$$ = $$4*2*1*3$$ = $$24$$ ( Divided by 6 , remainder will be 0)

Let k = 3

$$(k + 2)k(k – 1)(k + 1)$$ = $$5*3*2*4$$ = $$120$$ ( Divided by 6 , remainder will be 0)

Thus, the answer will be (A) 0
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Re: If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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AbdurRakib wrote:
If k is a positive integer, what is the remainder when (k + 2)(k^3 – k) is divided by 6 ?

A. 0
B. 1
C. 2
D. 3
E. 4

Let’s simplify the given expression:

(k + 2)(k^3 – k) = (k + 2)[k(k^2 - 1)] = (k + 2)(k)(k + 1)(k - 1)

Reordering the factors in the expression, we have:

(k - 1)(k)(k + 1)(k + 2), which is a product of 4 consecutive integers. Since the product of n consecutive integers is always divisible by n!, the product of 4 consecutive integers is always divisible by 4! = 24 and hence by 6. Thus, the remainder when (k + 2)(k^3 – k) is divided by 6 is 0.

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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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Hi All,

This question it perfect for TESTing VALUES (the approach that Abhishek009 used). The question is also based on a subtle Number Property rule about consecutive integers.

(K+2)(K^3 - K) can be rewritten as....
(K+2)(K)(K^2 - 1) =
(K+2)(K)(K+1)(K-1)

When multiplying numbers, the 'order' doesn't matter, so you can re-order those 4 'pieces' as...
(K-1)(K)(K+1)(K+2)

We're told that K is a positive integer, so what you have here is the product of 4 consecutive integers (they'll either all be positive or it's be 0-1-2-3) and EVERY 4 consecutive integers will include at least two multiples of 2 and a multiple of 3. By extension, this product will be some multiple of (2)(3) = 6, so when you divide the product by 6 you'll get a remainder of 0.

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Re: If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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Bunuel wrote:
AbdurRakib wrote:
If k is a positive integer, what is the remainder when (k + 2)(k^3 – k) is divided by 6 ?

A. 0
B. 1
C. 2
D. 3
E. 4

There is a rule saying that: The product of n consecutive integers is always divisible by n!.

$$(k + 2)(k^3 – k) = (k + 2)k(k^2 – 1) = (k + 2)k(k – 1)(k+1)=(k – 1)k(k+1)(k + 2)$$.

As we can see we have the product of 4 consecutive integers: (k – 1), k, (k+1), and (k + 2). Thus, the product will always be divisible by 4! = 24, which means that it will be divisible by 6 too.

Of course we can simply plug-in numbers: since PS question cannot have more than 1 correct answer, then for any k the answer must be the same. For example, if k = 1, then the product becomes 0. 0 divided by 6 gives the remainder of 0 (0 is a multiple of every integer, except 0 itself).

Hi Bunuel :)

can you please explain one thing. pleeeese :)

ok so you say we hav a product of 4 consecutive integers (k – 1), k, (k+1), and (k + 2) but that one K is not in the form of (k-1) it is just single K :) why ?

for example i have this (k – 1), k, (k+1), and (k + 2) and will take 4 and plug in

so i get (4 – 1), 4, (4+1), and (4 + 2) --- > 3, 4, 5, 6

but if I take 0 for K i get ( 0 – 1), 0, (0+1), and (0+ 2) ---> i get 0 and then remain is 6 ? thank you Math Expert V
Joined: 02 Sep 2009
Posts: 59712
Re: If k is a positive integer, what is the remainder when (k + 2)(k^3 – k  [#permalink]

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1
dave13 wrote:
Bunuel wrote:
AbdurRakib wrote:
If k is a positive integer, what is the remainder when (k + 2)(k^3 – k) is divided by 6 ?

A. 0
B. 1
C. 2
D. 3
E. 4

There is a rule saying that: The product of n consecutive integers is always divisible by n!.

$$(k + 2)(k^3 – k) = (k + 2)k(k^2 – 1) = (k + 2)k(k – 1)(k+1)=(k – 1)k(k+1)(k + 2)$$.

As we can see we have the product of 4 consecutive integers: (k – 1), k, (k+1), and (k + 2). Thus, the product will always be divisible by 4! = 24, which means that it will be divisible by 6 too.

Of course we can simply plug-in numbers: since PS question cannot have more than 1 correct answer, then for any k the answer must be the same. For example, if k = 1, then the product becomes 0. 0 divided by 6 gives the remainder of 0 (0 is a multiple of every integer, except 0 itself).

Hi Bunuel can you please explain one thing. pleeeese ok so you say we hav a product of 4 consecutive integers (k – 1), k, (k+1), and (k + 2) but that one K is not in the form of (k-1) it is just single K why ?

for example i have this (k – 1), k, (k+1), and (k + 2) and will take 4 and plug in

so i get (4 – 1), 4, (4+1), and (4 + 2) --- > 3, 4, 5, 6

but if I take 0 for K i get ( 0 – 1), 0, (0+1), and (0+ 2) ---> i get 0 and then remain is 6 ? thank you 0 is divisible by every integer (except 0 itself): 0/integer = integer. So, 0 divided by 6 gives the remainder of 0.
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