700 level question : GMAT Problem Solving (PS)
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# 700 level question

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Intern
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22 Aug 2010, 20:56
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Difficulty:

25% (medium)

Question Stats:

70% (01:26) correct 30% (00:14) wrong based on 33 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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22 Aug 2010, 21:12
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.

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23 Aug 2010, 01:26
shekar123 wrote:
What is the last digit of 3^987 ?

a.1

b.7

c.9

d.2

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15 Oct 2010, 08:26
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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17 Oct 2010, 18:15
B
The last digit for power of 3 is [3,9,7,1]. And 987 = 4k+3
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18 Oct 2010, 05:23
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)
Re: 700 level question   [#permalink] 18 Oct 2010, 05:23
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