Try to prove it!

Is there a multiple of 2008 that only has the digit 8 in its base 10 representation?

No Yes

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

3 solutions

Mark Hennings
Mar 6, 2017

By Fermat's Little Theorem , 1 0 250 1 ( m o d 251 ) 10^{250} \equiv 1 \pmod{251} , and hence 1 0 250 1 ( m o d 9 × 251 ) 10^{250} \equiv 1 \pmod{9\times251} , so that N = 8 9 ( 1 0 250 1 ) = 8 × 251 X = 2008 X N \; =\; \tfrac89(10^{250}-1) \; = \; 8\times251X \; = \; 2008X where X X is an integer. But N N is the integer whose decimal representation is a string of 250 250 8 8 s.

Indeed 1 0 50 1 ( m o d 251 ) 10^{50} \equiv 1 \pmod{251} , so the integer M M consisting of 50 50 8 8 s in a row will do as well.

Very nice that, instead of just proving the existence, you constructed such a number!

H K - 4 years, 3 months ago
H K
Mar 6, 2017

Consider the numbers 1, 11, 111, ... , 11...11 the last of which consists of 2009 digits 1. Since there are 2009 of these numbers, at least two of them have got to be congruent modulo 2008. The positive difference of two such congruent numbers is thus a multpile of 2008 and of the form 1...10...0 (some number of ones followed by some number of zeros). We will call this number A.

Since 2008 = 8×251, A is also divisible by 251. Furthermore, because 251 is clearly coprime to any power of 10, dividing A by an appropriate power of 10 produces a quotient of the form 11...11 which is still a multiple of 251. Multiplying this quotient by 8, we get a number with only the digit 8 in its base 10 representation that is divisible by 8×251 = 2008. The answer is yes.

The source of this question: Illinois Undergrad Math Contest 2008, Question 1.

H K - 4 years, 3 months ago

Simple, standard Pigeonhole Principle problem.

Sharky Kesa - 4 years, 3 months ago

We have,

2008 = 251 × 8 2008=251\times8

for a number to have only 8's in its decimal representation it should be of the form

x = 8 × ( 1 0 n 1 ) 9 x=8\times \dfrac{(10^{n}-1)}{9} where n 1 n\geq1

however,for it to be a multiple of 2008 2008 , ( 1 0 n 1 ) 9 \dfrac{(10^{n}-1)}{9} must be divisible by 251 251

since 251 251 is prime, by Fermat's Little theorem we have,

1 0 250 1 ( m o d 251 ) 10^{250}\equiv 1\pmod{251} ,or

1 0 250 1 0 ( m o d 251 ) 10^{250}-1\equiv 0\pmod{251}

1 0 250 1 0 ( m o d 251 ) 10^{250}-1\equiv 0\pmod{251}

1 0 250 1 9 0 ( m o d 251 ) \Rightarrow \dfrac{10^{250}-1}{9}\equiv 0\pmod{251} (since 251 251 and 9 9 are relatively prime)

thus we have,

8 × 1 0 250 1 9 0 ( m o d 2008 ) 8\times\dfrac{10^{250}-1}{9}\equiv 0\pmod{2008}

The answer is yes.

8 is not divisble by 2008

Anirudh Sreekumar - 4 years, 3 months ago

Log in to reply

you are right, i thought the question demanded the divisor of 2008. it's asking for multiple. Thanks

Achal Jain - 4 years, 3 months ago

But as you have mentioned 8 × 251 = 2008 8 \times 251 =2008 and 8 8 is a number which has only the digit 8 8 in it's decimal representation. Why complicate?

Achal Jain - 4 years, 3 months ago

@Achal Jain - I don't get what you mean??

Anirudh Sreekumar - 4 years, 3 months ago

The Question demands if there is a multiple which has only 8 8 in its decimal representation and 8 8 is such a number.

Achal Jain - 4 years, 3 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...