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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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\(x\) 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, 00:09.
Last edited by blitzkriegxX on 17 Dec 2018, 00:27, 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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New post 17 Dec 2018, 06:52
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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New post Updated on: 17 Dec 2018, 07:36
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, 07:02.
Last edited by blitzkriegxX on 17 Dec 2018, 07:36, 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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New post 17 Dec 2018, 07:10
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) \(y = 7\)
Nothing about y..
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 = \((-1)^{78y}=1\)
_________________

1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html


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

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New post 17 Dec 2018, 07:14
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) \(y = 7\)
Nothing about y..
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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New post 17 Dec 2018, 07:25
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. :)
_________________

1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html


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User avatar
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Joined: 28 Jun 2018
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CAT Tests
x and y are positive integers. What is the remainder when x is divided  [#permalink]

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New post 17 Dec 2018, 07:34
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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New post 17 Dec 2018, 07:44
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.
_________________

1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html


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Re: x and y are positive integers. What is the remainder when x is divided &nbs [#permalink] 17 Dec 2018, 07:44
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