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PS: what is the units digit of n ?

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PS: what is the units digit of n ? [#permalink] New post 29 Mar 2008, 21:11
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Came across this interesting question. Thought I will share with you guys....

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

Last edited by suntaurian on 29 Mar 2008, 21:24, edited 1 time in total.
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Re: PS: what is the units digit of n ? [#permalink] New post 29 Mar 2008, 21:17
[Edited]

The question was entered as (243)x(463)y initially. Please ignore this post.

Last edited by purefocus428 on 17 Apr 2008, 18:51, edited 1 time in total.
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Re: PS: what is the units digit of n ? [#permalink] New post 29 Mar 2008, 21:26
suntaurian wrote:
Came across this interesting question. Thought I will share with you guys....

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


Edited the question to properly indicate that its 243^x and 463^y.. Sorry about that...
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Re: PS: what is the units digit of n ? [#permalink] New post 29 Mar 2008, 21:34
purefocus428 wrote:
(243)x(463)y has the same units digit as 3x3y or 9xy.

(1) x + y = 7
x and y could be (0,7) or (2,5) which would give different answers
INSUFF

(2) x = 4
No way of knowing what y is,
INSUFF

(1 and 2) Since x+y = 7, the fact that x = 4 means that y is 3.
9(4)(3) gives a units digit of 8

(C)


Keep trying...Will post the OA after few others have tried as well...
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Re: PS: what is the units digit of n ? [#permalink] New post 29 Mar 2008, 22:47
suntaurian wrote:
Came across this interesting question. Thought I will share with you guys....

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


looking at unit digit
3^1 = 3
3^2 = 9
3^3 = 7
3^4 = 1
3^5 = 3
3^6 = 9

1) sufficient - any combinations of x and y will result in unit digit of 7
i.e. x=1 y=6; unit digit = 3*9=7
x=2, y=5; unit digit = 9*3=7
x=3, y=4; unit digit = 7*1=7
also vice versa

2) not sufficient, we need to know y in this case

I'd go with A
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Re: PS: what is the units digit of n ? [#permalink] New post 29 Mar 2008, 23:11
suntaurian wrote:
suntaurian wrote:
Came across this interesting question. Thought I will share with you guys....

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


Edited the question to properly indicate that its 243^x and 463^y.. Sorry about that...


A is suff since the last digit of either of the number is 3.
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Re: PS: what is the units digit of n ? [#permalink] New post 29 Mar 2008, 23:32
Ah, well that changes the question now that I know they are exponents :P

I would still do same approach where I believe that only the units digits make a difference.

(243)^x(463)^y has the same units digit as (3^x)(3^y) or 3^(x+y)

Since Statement 1 gives the value of x+y as seven, the answer is (A) since statement 2 doesn't help by itself.
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Re: PS: what is the units digit of n ? [#permalink] New post 29 Mar 2008, 23:33
suntaurian wrote:
purefocus428 wrote:
(243)x(463)y has the same units digit as 3x3y or 9xy.

(1) x + y = 7
x and y could be (0,7) or (2,5) which would give different answers
INSUFF

(2) x = 4
No way of knowing what y is,
INSUFF

(1 and 2) Since x+y = 7, the fact that x = 4 means that y is 3.
9(4)(3) gives a units digit of 8

(C)


Keep trying...Will post the OA after few others have tried as well...


Please ignore this post. Looks like I answered it at the same time the post was being corrected.
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Re: PS: what is the units digit of n ? [#permalink] New post 17 Apr 2008, 18:22
short way to do this is to realize the unit digit of each number is 3..

therefore the unit digit of N= 3^(x+y)=3^7=7

A it is..
Re: PS: what is the units digit of n ?   [#permalink] 17 Apr 2008, 18:22
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