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# x and y are positive integers. What is the remainder when x is divided

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x and y are positive integers. What is the remainder when x is divided  [#permalink]

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Updated on: 17 Dec 2018, 01:27
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Question Stats:

39% (01:26) correct 61% (01:21) wrong based on 117 sessions

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$$x$$ and $$y$$ are positive integers. What is the remainder when $$x$$ is divided by $$2^2$$?

(1) $$y = 7$$

(2) $$x = 3^{78y}$$

Originally posted by blitzkriegxX on 17 Dec 2018, 01:09.
Last edited by blitzkriegxX on 17 Dec 2018, 01:27, edited 2 times in total.
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Re: x and y are positive integers. What is the remainder when x is divided  [#permalink]

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17 Dec 2018, 07:52
Hi i would like to know why ans is B here...since third digit is missing how can one determine what wud b the reminder? please explain
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x and y are positive integers. What is the remainder when x is divided  [#permalink]

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Updated on: 17 Dec 2018, 08:36
Hi SWAPNILP
I don't want to provide the official explanation soo soon. Wanted to give a chance for others to think as well.

Also it is not a digit. It is (78 multiplied by y)

But here is a very big clue for you-
In Gmat a lot of problems depend on pattern recognision. So check the patterns when powers of 3 are divided by 4 mentioned in the question stem.

Edit- oops! Looks like chetan2u gave an awesome response (maybe a little too soon :D).

Originally posted by blitzkriegxX on 17 Dec 2018, 08:02.
Last edited by blitzkriegxX on 17 Dec 2018, 08:36, edited 4 times in total.
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x and y are positive integers. What is the remainder when x is divided  [#permalink]

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17 Dec 2018, 08:10
1
1
SWAPNILP wrote:
Hi i would like to know why ans is B here...since third digit is missing how can one determine what wud b the reminder? please explain

Quote:
$$x$$ and $$y$$ are positive integers. What is the remainder when $$x$$ is divided by $$2^2$$?

(1) $$y = 7$$

(2) $$x = 3^{78y}$$

Hi..
78y is not a 3-digit number but 78*y and you can say this because it is given that y is a positive integer.

so let us see the question..

(1) $$y = 7$$
insuff

(2) $$x = 3^{78y}$$
Now, you should check few multiples of 3 and you will find a pattern ..
3^1 divided by 4 leaves 3 as remainder
3^2 =9 leaves 1 as remainder
3^3=27 leaves 3 as remainder and so on.. so pattern is 3,1,3,1...
$$x = 3^{78y}$$, and this has an even power 78y, so answer will be that remainder is 1...
sufff

B

Ofcourse other way is binomial expansion.. $$x = 3^{78y}=(4-1)^{78y}$$...
In expansion. all terms except $$(-1)^{78y}$$ will be multiple of 4, so remainder = $$(-1)^{78y}=1$$
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Re: x and y are positive integers. What is the remainder when x is divided  [#permalink]

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17 Dec 2018, 08:14
chetan2u wrote:
SWAPNILP wrote:
Hi i would like to know why ans is B here...since third digit is missing how can one determine what wud b the reminder? please explain

Quote:
$$x$$ and $$y$$ are positive integers. What is the remainder when $$x$$ is divided by $$2^2$$?

(1) $$y = 7$$

(2) $$x = 3^{78y}$$

Hi..
78y is not a 3-digit number but 78*y and you can say this because it is given that y is a positive integer.

so let us see the question..

(1) $$y = 7$$
insuff

(2) $$x = 3^{78y}$$
Now, you should check few multiples of 3 and you will find a pattern ..
3^1 divided by 4 leaves 3 as remainder
3^2 =9 leaves 1 as remainder
3^3=27 leaves 3 as remainder and so on.. so pattern is 3,1,3,1...
$$x = 3^{78y}$$, and this has an even power 78y, so answer will be that remainder is 1...
sufff

B

Ofcourse other way is binomial expansion.. $$x = 3^{78y}=(4-1)^{78y}$$...
In expansion. all terms except $$(-1)^{78y}$$ will be multiple of 4, so remainder = $$(-1)^{78y}=1$$

thanks chetan2u
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Re: x and y are positive integers. What is the remainder when x is divided  [#permalink]

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17 Dec 2018, 08:25
1
blitzkriegxX wrote:
Hi SWAPNILP
I don't want to provide the official explanation soo soon. Wanted to give a chance for others to think as well.

But here is a very big clue for you-
In Gmat a lot of problems depend on pattern recognision. So check the patterns when powers of 3 are divided by 4 mentioned in the question stem.

Edit- oops! Looks like chetan2u gave an awesome response (maybe a little too soon :D).

Hi blitzkriegxX,
I wanted to clarify on 78y as a 3-digit number or a product 78*y.
I think in that process, the solution too came out.
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x and y are positive integers. What is the remainder when x is divided  [#permalink]

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17 Dec 2018, 08:34
chetan2u wrote:

Hi blitzkriegxX,
I wanted to clarify on 78y as a 3-digit number or a product 78*y.
I think in that process, the solution too came out.

Hey chetan2u
No worries.
And you're right. I did mean the product 78*y.
Is this how the product part is expressed in official gmat questions?
Or should I edit my question to 78*y ?
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Re: x and y are positive integers. What is the remainder when x is divided  [#permalink]

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17 Dec 2018, 08:44
1
blitzkriegxX wrote:
chetan2u wrote:

Hi blitzkriegxX,
I wanted to clarify on 78y as a 3-digit number or a product 78*y.
I think in that process, the solution too came out.

Hey chetan2u
No worries.
And you're right. I did mean the product 78*y.
Is this how the product part is expressed in official gmat questions?
Or should I edit my question to 78*y ?

Hi..
you are perfectly fine with the way you have written, 78y would mean product unless specified otherwise, and y as positive integer also makes it much more clearer, because if 78y were a 3-digit number, y would be a digit, and not integer.
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Re: x and y are positive integers. What is the remainder when x is divided  [#permalink]

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20 Jan 2019, 17:24
blitzkriegxX
What is the question source?

chetan2u
Quote:
I wanted to clarify on 78y as a 3-digit number or a product 78*y.
I think in that process, the solution too came out.

By this you mean the solution is B as well? 3^{(195·4)+1}, 3^{(195·4)+2}
I confused the exponents and thought it was a third digit and so selected C
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Re: x and y are positive integers. What is the remainder when x is divided  [#permalink]

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20 Jan 2019, 20:57
philipssonicare wrote:
blitzkriegxX
What is the question source?

chetan2u
Quote:
I wanted to clarify on 78y as a 3-digit number or a product 78*y.
I think in that process, the solution too came out.

By this you mean the solution is B as well? 3^{(195·4)+1}, 3^{(195·4)+2}
I confused the exponents and thought it was a third digit and so selected C

Yes, I mean answer is B..
Tye initial statement says that X and y are integers. So if y is an integer, 78y would mean 78*y.
Had it been given that y is a digit, it could have been a 3-digit number. But, here too it is generally given that 78y is a 3-digut number or 78y is an integer etc
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Re: x and y are positive integers. What is the remainder when x is divided  [#permalink]

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20 Jan 2019, 21:03
chetan2u
Apologies, I wasn't clear. I meant in the scenario that it is a 3 digit number of 78[y]. Hence the $$3^{(195·4)+1}, 3^{(195·4)+2}$$ when y =1 and y=2 and so on, is the answer still B?
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Re: x and y are positive integers. What is the remainder when x is divided  [#permalink]

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20 Jan 2019, 21:38
philipssonicare wrote:
chetan2u
Apologies, I wasn't clear. I meant in the scenario that it is a 3 digit number of 78[y]. Hence the $$3^{(195·4)+1}, 3^{(195·4)+2}$$ when y =1 and y=2 and so on, is the answer still B?

No, then the answer will be C.
Reason is that we are dividing X by 2^*2 or 4..
Now X is 3 to the power something.

When you divide 3 with odd power that is 3^1 or 3^3 or 3^781, the remainder will always be 3..
But with even power that is 3^2 or 3^782, the remainder will be 1..
So we have to know whether y is even or odd..

Statement I tells us that it is odd, hence remainder is 1

Thus combined the two statements are sufficient

C
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Re: x and y are positive integers. What is the remainder when x is divided  [#permalink]

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21 Jan 2019, 05:20
But how one can know whether 78y (a three digit no.) or 78*y? It's second time that i got confused in these problems.
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Re: x and y are positive integers. What is the remainder when x is divided  [#permalink]

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21 Jan 2019, 05:24
1
Vikas1211 wrote:
But how one can know whether 78y (a three digit no.) or 78*y? It's second time that i got confused in these problems.

If it were a three-digit number it would have been explicitly mentioned. Otherwise, 78y ALWAYS means 78*y.
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Re: x and y are positive integers. What is the remainder when x is divided   [#permalink] 21 Jan 2019, 05:24
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