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What is the remainder of 3^19 when divided by 10?

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What is the remainder of 3^19 when divided by 10? [#permalink] New post 27 May 2006, 21:53
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76% (01:39) correct 24% (00:45) wrong based on 130 sessions
What is the remainder of 3^19 when divided by 10?

A. 0
B. 1
C. 5
D. 7
E. 9
[Reveal] Spoiler: OA
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 [#permalink] New post 27 May 2006, 22:17
I got D.
You can use the Mod function to solve this kind of problem.
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 [#permalink] New post 27 May 2006, 22:17
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3^19=3^4*3^4*3^4*3^4*3^3
3^4=81/10 has rmainder 1
3^3=27/10 has remainder 7
1*1*1*1*7=7
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 [#permalink] New post 27 May 2006, 22:21
D it is...

3 ^ 16 will give remainder 1
3 ^ 3 will give remainder 7

so 7 * 1= 7
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 [#permalink] New post 31 May 2006, 17:12
I am getting 7 - D

brute force to find the pattern in this case 39713971 and then just count to the 19th power we get 7
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How do you solve? [#permalink] New post 07 Feb 2014, 16:06
what is the remainder when 3^19 is divided by 10

The answer is 7 but I don't understand why. Is it as simple as 10-3?
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Re: How do you solve? [#permalink] New post 07 Feb 2014, 16:15
Luning1 wrote:
what is the remainder when 3^19 is divided by 10

The answer is 7 but I don't understand why. Is it as simple as 10-3?


Just look at how power of 3 works..

3^1 = 3
3^2 = 9
3^3 = 27
3^4 = 81
3^5 =243
.....

You'd notice that unit digit repeats itself after every 4 numbers.. In other words, 3^19 will have same unit number as 3^15, or 3^11 or 3^7 or 3^3....Which tells us, 3^19 has unit digit as 7..

Now, we know that any number with unit digit as 7 when divided by 10 will give u 7....

PS: Wrong section... Moving it to PS..
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Re: How do you solve? [#permalink] New post 07 Feb 2014, 18:29
what is the remainder when 3^19 is divided by 10?

Units digit of 3^n (n = 1,2,3....) follows the following pattern: 3,9,7,1,3,9,7,1...
When divided by 10, the remainders would be 3,9,7,1,3,9,7,1...
3^19/10 -> remainder = 7
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Re: What is the remainder of 3^19 when divided by 10? [#permalink] New post 10 Sep 2014, 20:52
we are interested in last digit of 3^19, so we should find circulation

3*1=3
3*2=9
3*3=7
3*4=1
3*5=3
circulation is 4, that means 19/4=4 with 3 remainder, so 7 is last digit. Dividing 10 gives us 7/10, so 7 is remainder

D
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What is the remainder of 3^19 when divided by 10? [#permalink] New post 12 Sep 2014, 10:48
I got D, and this is how I solved:
I looked for patterns:
^2 - units digit 9
^3 - units digit 7
^4 - units digit 1
^5 - units digit 3

hence, we can see that when raised to a power which is multiple of 4, the units digit is 1, and when to an even power not multiple of 4, the units digit is 9
and we can then see:
^16 - units digit 1, or
^18 - units digit 9
and ^19 - units digit 7

therefore, when divided by 10, the remainder must be 7
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Re: What is the remainder of 3^19 when divided by 10? [#permalink] New post 30 Sep 2014, 22:23
For 3^1, remainder = 3

3^2, remainder = 9

3^3, remainder = 7

3^4, remainder = 1

& so on........

For 3^{19}, remainder = 7

Answer = D
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Re: What is the remainder of 3^19 when divided by 10?   [#permalink] 30 Sep 2014, 22:23
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