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If a^b=c, what is the units digit of c?

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Intern
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If a^b=c, what is the units digit of c? [#permalink] New post 24 Mar 2011, 16:56
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B
C
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  5% (low)

Question Stats:

58% (01:19) correct 41% (00:46) wrong based on 12 sessions
Hey guys,

How would you solve the following DS question:

If a^b=c, what is the units digit of c?

1. a^2 – 36 = 0
2. b is a positive integer.

Thanks
[Reveal] Spoiler: OA
Director
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Re: unit digit [#permalink] New post 24 Mar 2011, 17:04
1. Insufficient.
b is not known.
If b = 0 then the answer is 1
If b>0 then the answer is 6

2. Insufficient

1) + 2) Sufficient.
a = -6 or +6 and b > 0
The answer is 6

CAUTION : The catch here b should be "integer" which is true from 2)

rait_m wrote:
Hey guys,

How would you solve the following DS question:

If a^b=c, what is the units digit of c?

1. a^2 – 36 = 0
2. b is a positive integer.

Thanks
Director
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Posts: 771
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Re: unit digit [#permalink] New post 24 Mar 2011, 18:08
1. Not sufficient
As we dont know anything about b ,its not possible to determine a^b

2. Not sufficient

Here we dont know anything about a.so its not possible to determine a^b

together we know

a = +6 or a = -6 and b is positive integer,

this is enough to say c = 6 .

Answer C.
Director
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Re: unit digit [#permalink] New post 25 Mar 2011, 05:38
rait_m wrote:
Hey guys,

How would you solve the following DS question:

If a^b=c, what is the units digit of c?

1. a^2 – 36 = 0
2. b is a positive integer.

Thanks


(1) a= ±6, nothing about b. if b =0 unit digit will 1, and b= 1 unit digit will 6 Insuff.
(2) b is positive so b>0, any power of 6 gives 6 as unit digit.
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Re: unit digit   [#permalink] 25 Mar 2011, 05:38
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