cyclicity : GMAT Problem Solving (PS)
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# cyclicity

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Manager
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cyclicity [#permalink]

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20 Aug 2010, 22:56
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25% (01:22) correct 75% (00:18) wrong based on 28 sessions

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What is the last digit of 3^987 ?
• 1
• 3
• 6
• 7
• 9
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Re: cyclicity [#permalink]

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21 Aug 2010, 00:19
bibha wrote:
What is the last digit of 3^987 ?
• 1
• 3
• 6
• 7
• 9

3 has cyclicity of 4.......

so answer is 7 (d)
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Re: cyclicity [#permalink]

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21 Aug 2010, 01:54
D

reason, 3 has cyclicity of 4.....
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Re: cyclicity [#permalink]

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21 Aug 2010, 04:40
I know about it. But, can you please elaborate a little?
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Re: cyclicity [#permalink]

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21 Aug 2010, 12:39
it would be 3 raised to the power 3 which would have 7 as it's unit place
Cyclicity of 3 --
3^1=3,
3^2=9,
3^3=27,
3^4=81,
3^5=243,
.....So on
see the unit place it is 3,9,7,1,3,9,7,1.......and repeats after every 4 power
so divide 987 by 4 remainder is 3. Therefore, 3^3 has unit place digit as 7

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Re: cyclicity   [#permalink] 21 Aug 2010, 12:39
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# cyclicity

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