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If 243^x*463^y = n, where x and y are positive integers

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If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 02 Oct 2010, 04:44
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If \(243^x*463^y =n\) , where x and y are positive integers, what is the units digit of n?

(1) x + y = 7

(2) x = 4
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 02 Oct 2010, 04:56
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If \(243^x*463^y =n\), where x and y are positive integers, what is the units digit of n?

The units digit of \(243^x\) is the same as the units digit of \(3^x\) and similarly the units digit of \(463^y\) is the same as the units digit of \(3^y\), so the units digit of \(243^x*463^y\) equals to the units digit of \(3^x*3^y=3^{x+y}\). So, knowing the value of \(x+y\) is sufficient to determine the units digit of \(n\).

(1) \(x + y = 7\). Sufficient. (As cyclicity of units digit of \(3\) in integer power is \(4\), units digit of \(3^7\) would be the same as of units digit of \(3^3\) which is \(7\))

(2) \(x=4\). No info about \(y\). Not sufficient.

Answer: A.

Hope it helps.
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 03 Oct 2010, 09:50
Yup!! A it is....equation can be treated like 3^x*3^y hence (x+y)'s value can provide us the last digit...
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 03 Mar 2014, 05:18
I have a doubt. Cyclicity of unit digit of 3 is 4. Hence we know that every fourth power of 3 (3^4, 3^8, 3^12) will have the same unit digit, 1. Hence when option B says x = 4, knowing that x and y are positive integers, we know that xy will be a multiple of 4. Unit digit of 3^4k is always 1 isn't it? Shouldn't this be sufficient information?

Shouldn't the answer be D?
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 03 Mar 2014, 05:21
siriusblack1106 wrote:
I have a doubt. Cyclicity of unit digit of 3 is 4. Hence we know that every fourth power of 3 (3^4, 3^8, 3^12) will have the same unit digit, 1. Hence when option B says x = 4, knowing that x and y are positive integers, we know that xy will be a multiple of 4. Unit digit of 3^4k is always 1 isn't it? Shouldn't this be sufficient information?

Shouldn't the answer be D?


I think you are missing that \(3^x*3^y=3^{x+y}\), so the exponent is x+y not xy.

Does this make sense?
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 03 Mar 2014, 05:26
Yes! Can't believe I just made that mistake. Such mistakes are gonna cost me. :/
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 03 Mar 2014, 05:28
siriusblack1106 wrote:
Yes! Can't believe I just made that mistake. Such mistakes are gonna cost me. :/


Yes, careless errors are the #1 cause of score drops on the GMAT! They cause you to miss easier questions, hurting your score a lot more than not know how to solve the harder ones. So, be more careful, before you submit your answer, double-check that it’s the answer to the proper question.
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 18 May 2014, 09:53
1
243 = 3^5

463 ends with a 3. So we have to know how many times we will multiply 3's at the end of each numbers.

1) 7 times - SUF
2) we dont know Y - INSUF

Choose (a)
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 02 Nov 2014, 18:48
Bunuel wrote:
If \(243^x*463^y =n\), where x and y are positive integers, what is the units digit of n?

The units digit of \(243^x\) is the same as the units digit of \(3^x\) and similarly the units digit of \(463^y\) is the same as the units digit of \(3^y\), so the units digit of \(243^x*463^y\) equals to the units digit of \(3^x*3^y=3^{x+y}\). So, knowing the value of \(x+y\) is sufficient to determine the units digit of \(n\).

(1) \(x + y = 7\). Sufficient. (As cyclicity of units digit of \(3\) in integer power is \(4\), units digit of \(3^7\) would be the same as of units digit of \(3^3\) which is \(7\))

(2) \(x=4\). No info about \(y\). Not sufficient.

Answer: A.

Hope it helps.


Hi Bunuel,

But surely shouldn't it matter if 3 is raised to 1 and or 6? Meaning, if it's 3^3 + 3^4 = 7 + 1 = 8. But, if its 3^2+3^5 = 9 + 3 = 12, units of 2. Doesn't that yield insufficient?
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 03 Nov 2014, 01:50
russ9 wrote:
Bunuel wrote:
If \(243^x*463^y =n\), where x and y are positive integers, what is the units digit of n?

The units digit of \(243^x\) is the same as the units digit of \(3^x\) and similarly the units digit of \(463^y\) is the same as the units digit of \(3^y\), so the units digit of \(243^x*463^y\) equals to the units digit of \(3^x*3^y=3^{x+y}\). So, knowing the value of \(x+y\) is sufficient to determine the units digit of \(n\).

(1) \(x + y = 7\). Sufficient. (As cyclicity of units digit of \(3\) in integer power is \(4\), units digit of \(3^7\) would be the same as of units digit of \(3^3\) which is \(7\))

(2) \(x=4\). No info about \(y\). Not sufficient.

Answer: A.

Hope it helps.


Hi Bunuel,

But surely shouldn't it matter if 3 is raised to 1 and or 6? Meaning, if it's 3^3 + 3^4 = 7 + 1 = 8. But, if its 3^2+3^5 = 9 + 3 = 12, units of 2. Doesn't that yield insufficient?


Are you sure you are reading the question correctly? It's 243^x*463^y, 243^x multiplied by 463^y not 243^x + 463^y...
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 28 Jul 2016, 15:01
Well I don't agree the answer should be indeed D.
Option 1 suggests x+y=7 this can have multiple x and y combinations like (1,6) (2,5) (4,3) and so on so the units digit of 243^x and 463^y will differ .
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 28 Jul 2016, 18:37
sandeep211986 wrote:
Well I don't agree the answer should be indeed D.
Option 1 suggests x+y=7 this can have multiple x and y combinations like (1,6) (2,5) (4,3) and so on so the units digit of 243^x and 463^y will differ .


Hey Buddy,

All the combinations, for x+y=7, will yield the same units digit. Consider the following
x=1,y=6
243*463*463*463*463*463*463 ~~ To find units digit we just need 3^1 * 3^6 = 3^7, i.e 7 (units digit of 2187)

Same goes with other combinations. 3^7 ends up deciding the units digit.

Should the question would have been something like, 245^x * 463^y = n, the combinations of different values of x & y would have yielded different units digits.

Hope that clears
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 20 Dec 2016, 15:25
amandeep_k wrote:
If (243) x(463) y = n, where x and y are positive integers, what is the units digit of n?

(1) x + y = 7

(2) x = 4


I can't get how the answer can be A.

Had it been \(243^x * 463^y\). In this case we'll get same base 3 and we can add the powers

\(3^0*3^7\)
\(3^1*3^6\)
\(3^2*3^5\)
...

In each case we'll get \(3^7\) which units digit we can identify. That that will be sufficient.

But we have 3*x*3*y = 9*x*y which can take any values. Not sufficient. I can't get the idea how answer can be A.

Please correct me if I'm wrong.
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 20 Dec 2016, 15:39
vitaliyGMAT wrote:
amandeep_k wrote:
If (243) x(463) y = n, where x and y are positive integers, what is the units digit of n?

(1) x + y = 7

(2) x = 4


I can't get how the answer can be A.

Had it been \(243^x * 463^y\). In this case we'll get same base 3 and we can add the powers

\(3^0*3^7\)
\(3^1*3^6\)
\(3^2*3^5\)
...

In each case we'll get \(3^7\) which units digit we can identify. That that will be sufficient.

But we have 3*x*3*y = 9*x*y which can take any values. Not sufficient. I can't get the idea how answer can be A.

Please correct me if I'm wrong.


Hi, You are right. my question was wrong. Sorry for the inconvenience. :cry:
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Re: If 243^x*463^y = n, where x and y are positive integers  [#permalink]

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New post 18 Dec 2017, 02:58
1
A.
If we map the cyclicity of 3--> with x + y values --> 3 and 4/ 4 and 3/ 1 and 6; 6 and 1/ 5 and 2; 2 and 5 [3,9,7,1,3,9,7,1...] --> the answer is always 7. [7*1 = 7; 3*9 = _7..so on]

St 2 . No value for y
Re: If 243^x*463^y = n, where x and y are positive integers &nbs [#permalink] 18 Dec 2017, 02:58
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