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An “Armstrong number” is an n-digit number that is equal to the sum of

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An “Armstrong number” is an n-digit number that is equal to the sum of  [#permalink]

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New post 21 Sep 2019, 14:21
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A
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Question Stats:

67% (02:42) correct 33% (02:48) wrong based on 48 sessions

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An “Armstrong number” is an n-digit number that is equal to the sum of the nth powers of its individual digits. For example, 153 is an Armstrong number because it has 3 digits and 1^3 + 5^3 + 3^3 = 153. What is the digit k in the Armstrong number 1,6k4 ?

A. 2
B. 3
C. 4
D. 5
E. 6


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Re: An “Armstrong number” is an n-digit number that is equal to the sum of  [#permalink]

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New post 21 Sep 2019, 22:00
1⁴+6⁴+k⁴+4⁴=1604+10k
1+1296+k⁴+256=1604+10k
k⁴-10k=1604-1553=51
By replacing the options with k, 3 would be the answer.
3⁴-30=51
Option B

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Re: An “Armstrong number” is an n-digit number that is equal to the sum of  [#permalink]

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New post 22 Sep 2019, 02:02
digit 16k4 ; 1^4+6^4+k^4+4^4
so test with each option value
at k=3 we get 1634 =1^4+6^4+3^4+4^4
IMO B


gmatt1476 wrote:
An “Armstrong number” is an n-digit number that is equal to the sum of the nth powers of its individual digits. For example, 153 is an Armstrong number because it has 3 digits and 1^3 + 5^3 + 3^3 = 153. What is the digit k in the Armstrong number 1,6k4 ?

A. 2
B. 3
C. 4
D. 5
E. 6


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Re: An “Armstrong number” is an n-digit number that is equal to the sum of  [#permalink]

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New post 22 Sep 2019, 05:15
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gmatt1476 wrote:
An “Armstrong number” is an n-digit number that is equal to the sum of the nth powers of its individual digits. For example, 153 is an Armstrong number because it has 3 digits and 1^3 + 5^3 + 3^3 = 153. What is the digit k in the Armstrong number 1,6k4 ?

A. 2
B. 3
C. 4
D. 5
E. 6

PS36302.01


\(1^4+6^4+k^4+4^4=16k4\)

The units digits of \(1^4\), \(6^4\), and \(4^4\) are 1, 6, and 6, respectively. Since 1+6+6=13, the units digit of \(k^4\) must be 4-3=1.

The only answer choice whose 4th power has a unit digit of 1 is B.

Answer: B
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Re: An “Armstrong number” is an n-digit number that is equal to the sum of   [#permalink] 22 Sep 2019, 05:15
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