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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 \(7\))

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

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11 Sep 2013, 00:57

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

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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?

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.

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.
_________________

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

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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?

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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05 Jan 2016, 12:15

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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

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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 .

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

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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

gmatclubot

Re: If 243^x*463^y = n, where x and y are positive integers
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28 Jul 2016, 18:37

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