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What is the units digit of 13^2003 ?

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What is the units digit of 13^2003 ?  [#permalink]

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New post 19 Mar 2019, 02:35
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A
B
C
D
E

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  15% (low)

Question Stats:

75% (00:54) correct 25% (01:14) wrong based on 81 sessions

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Re: What is the units digit of 13^2003 ?  [#permalink]

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New post 19 Mar 2019, 02:42
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Bunuel wrote:
What is the units digit of \(13^{2003}\)?

(A) 1
(B) 3
(C) 7
(D) 8
(E) 9


Units digit of 3 has a cycle of 4

Hence units digit of \(13^{2003}\) ==> \(3^{2000}\)*\(3^{3}\)==>7

Hence C
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Re: What is the units digit of 13^2003 ?  [#permalink]

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New post 19 Mar 2019, 04:58
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Top Contributor
Bunuel wrote:
What is the units digit of \(13^{2003}\)?

(A) 1
(B) 3
(C) 7
(D) 8
(E) 9


Look for a pattern

13^1 = 13
13^2 = (13)(13) = ---9 [aside: we need not determine the other digits. All we care about is the units digit]
13^3 = (13)(13^2) = (13)(---9) = ----7
13^4 = (13)(13^3) = (13)(---7) = ----1
13^5 = (13)(13^4) = (13)(---1) = ----3

NOTICE that we're back to where we started.
13^5 has units digit 3, and 13^1 has units digit 3
So, at this point, our pattern of units digits keep repeating 3, 9, 7, 1, 3, 9, 7, 2, . . .
We say that we have a "cycle" of 4, which means the digits repeat every 4 powers.

So, we get:
13^1 = --3
13^2 = ---9
13^3 = ----7
13^4 = ----1
13^5 = ----3
13^6 = ---9
13^7 = ----7
13^8 = ----1
13^9 = ----3
13^10 = ----9
etc.

Notice that when the exponent is a MULTIPLE of 4 (4, 8, 12, 16, ...), the units digit will be 1
Since 2000 is a MULTIPLE of 4, we know that the units digit of 13^2000 will be 1
Continuing with the pattern:
13^2001 = --3
13^2002 = ---9
13^2003 = ----7


Answer: C

Here's an article I wrote on this topic (with additional practice questions): https://www.gmatprepnow.com/articles/un ... big-powers

Cheers,
Brent
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Re: What is the units digit of 13^2003 ?  [#permalink]

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New post 19 Mar 2019, 10:39
Bunuel wrote:
What is the units digit of \(13^{2003}\)?

(A) 1
(B) 3
(C) 7
(D) 8
(E) 9


\(13^{2003}\)

13^2000 *13^3
cyclicty = 13 is 4 so 13
13^3 = 7
IMO C
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Re: What is the units digit of 13^2003 ?   [#permalink] 19 Mar 2019, 10:39
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