What is the Divisibility Rule for the Reflection of an Integer?

In summary, the conversation revolved around the reflection of integers and its relation to divisibility. The participants discussed a problem from a practice GRE exam which involved finding the difference between a five-digit integer and its reflection, and determining which number it must be divisible by. They arrived at the conclusion that the answer is always divisible by 9, as the digit sum and the original number are equivalent modulo nine. They also discussed the usefulness of this property in various number tricks and calculations.
  • #1
cepheid
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"Reflection of an Integer"

I haven't encountered this before. I'm not sure how to approach it. At this point it's not even clear to me why the result should only be divisible by one number in *every* case.

The reflection of a positive integer is obtained by reversing its
digits. For example, the reflection of 321 would be 123. The
difference between a 5 digit integer and its reflection must be
divisible by which of the following?

A. 2
B. 4
C. 5
D. 6
E. 9
 
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  • #2
The number abcde is really [tex]10^{4}a + 10^{3}b + 10^{2}c + 10^{1}d + 10^{0}e [/tex]. You know what to do...
 
  • #3
so the difference becomes...

10^4 (a-e) + 10^3 (b-d) + 10^2 (c-c) + 10 (d-b) + (e-a)

= (10^4 - 1)(a-e) + (10^3 - 10)(b-d)

= 9999(a-e) + 990(b-d)

I guess that means the answer is divisible by 9. Did I do this right?
 
  • #4
Yes.
 
  • #5
That's it. Of course, with multiple choice looking for counterexamples might even be faster... heh.
 
  • #6
Hi Cepheid,

I find such problems interesting. Where did you find it?
Do you have a website for it?
 
  • #7
Edgardo,

This is a problem from a practice GRE exam. These exams are administered by ETS. However, I think that such problems are also common in math contests that are intended for junior high/high school students. Try a search for math contests on the net.
 
  • #8
cepheid said:
Edgardo,

This is a problem from a practice GRE exam. These exams are administered by ETS. However, I think that such problems are also common in math contests that are intended for junior high/high school students. Try a search for math contests on the net.

this is in the math GRE?
 
  • #9


ice109 said:
this is in the math GRE?

Yes it is, I got it in the practice section right now as the third question, and if you don't know your first 10 questions are the most important (28 total, 45 mins to complete). Suffice to say this test is going to kick my *** at 12PM tomorrow... but hey, I'm a psych major.
 
  • #10


And note that this is true not only for 5 digit numbers, it's true for all natural numbers.

For any natural number the digit sum and the original number are equivalent modulo nine. Since the number and it's reflection both have the same digit sum it follows that there are equal modulo 9, hence their differnece must be a multiple of nine.
 
  • #11


ice109 said:
this is in the math GRE?

pwrstick said:
Yes it is

Somehow I doubt that such questions are asked in the GRE math *subject test*. This problem was from the math (quantitative reasoning) section of a GRE *general* test (which, I think, is what pwrstick was talking about).
 
  • #12


uart said:
For any natural number the digit sum and the original number are equivalent modulo nine.

How do you know this? I mean, I convinced myself of it by figuring that the difference between the number and the sum of the digits can be expressed as:

Sum over all digits{ [(some power of ten) - 1]digit }

Therefore the difference between the actual number and its digit sum will always be a multiple of nine, hence they are equivalent modulo nine.

Is there a simpler/more obvious way of explaining your statement, though?
 
  • #13


Any power of ten is congruent to 1 (mod 9).
 
  • #14


cepheid said:
How do you know this?

Just elaborating on what Adrian said.

[tex]10^n = 9*(\underbrace{111...1}_{\mbox{n ones}}) + 1 = 9 k_n +1[/tex].

So the quantity represented by the nth digit (in a decimal number) is [itex]10^n d_n = 9k_n d_n + d_n[/itex]. That is, for each digit the quantity represented by that digit is congruent to the digit itself (mod 9).

Cepheid, this property is used in a wide variety of "number tricks" and was used for centuries before computers and calculators as a quick easy "checksum" for testing the integrety of long hand calculations. See casting out nines : http://en.wikipedia.org/wiki/Casting_out_nines" [Broken]
 
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  • #15


Hey uart,

Thanks for the link and for adding something useful to a dredged up thread from a year ago.
 

What is the definition of "reflection of an integer"?

The reflection of an integer is the negative of that integer. For example, the reflection of 5 is -5.

How is the reflection of an integer calculated?

The reflection of an integer can be calculated by multiplying the integer by -1.

What is the purpose of finding the reflection of an integer?

The reflection of an integer can be used in various mathematical equations and operations, such as finding the opposite direction or value in a number line, or solving equations involving negative numbers.

What is the difference between a reflection and a negation of an integer?

A reflection of an integer is the negative of the integer, while a negation of an integer is simply changing the sign of the integer without changing its value. For example, the negation of 5 is -5, while the reflection of 5 is also -5.

Can the reflection of an integer be a decimal or fraction?

No, the reflection of an integer will always result in a whole number. If a decimal or fraction is reflected, it will result in a different number that is not an integer.

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