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What is the last digit of 3^987 ?

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Intern
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Joined: 27 Sep 2009
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What is the last digit of 3^987 ? [#permalink]

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New post 22 Aug 2010, 21:56
00:00
A
B
C
D
E

Difficulty:

  15% (low)

Question Stats:

74% (00:31) correct 26% (00:14) wrong based on 39 sessions

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What is the last digit of 3^987 ?

A. 1

B. 7

C. 9

D. 2
[Reveal] Spoiler: OA

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Senior Manager
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Status: Time to step up the tempo
Joined: 24 Jun 2010
Posts: 404

Kudos [?]: 249 [0], given: 50

Location: Milky way
Schools: ISB, Tepper - CMU, Chicago Booth, LSB
Re: What is the last digit of 3^987 ? [#permalink]

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New post 22 Aug 2010, 22:12
shekar123 wrote:
What is the last digit of 3^987 ?

a.1

b.7

c.9

d.2


Numbers 2, 3, 7 and 8 has a cyclicity of 4. Hence every 1st, 5th, 9th and so on power of 2, 3, 7 and 8 has the same unit digit.

Here the power is 987. Divide it by 4, the remainder is 3. Hence the last digit of 3 ^ 987 is the same as 3 ^ 3 -- 7.

Answer is B (7).
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Re: What is the last digit of 3^987 ? [#permalink]

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New post 23 Aug 2010, 02:26
shekar123 wrote:
What is the last digit of 3^987 ?

a.1

b.7

c.9

d.2



700 level................I doubt...please check...!!!
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Manager
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Re: What is the last digit of 3^987 ? [#permalink]

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New post 15 Oct 2010, 09:26
For these types of ques ..just use the cyclicity. i.e 4 for 3..
3
9
7
1
so when power is divided by 4..we get 3 as rem..
so ans will be 7

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Re: What is the last digit of 3^987 ? [#permalink]

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New post 17 Oct 2010, 19:15
B
The last digit for power of 3 is [3,9,7,1]. And 987 = 4k+3
So answer is 7.
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Re: What is the last digit of 3^987 ? [#permalink]

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New post 18 Oct 2010, 06:23
3 has a cyclicity of 4
3^1=3
3^2=9
3^3=27
3^4=81
3^5=243
987/4=246 and rem 3
so we have 7
(B)

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Re: What is the last digit of 3^987 ?   [#permalink] 18 Oct 2010, 06:23
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What is the last digit of 3^987 ?

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