Maths Olympiad Prep

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Number theory Difficulty 6.2 National olympiad Prove it

2412 \cdot 41 digits are written along the circumference, and it is known that if we start from a certain position and read these digits in a clockwise direction, the resulting 1953-digit number is divisible by 27. Prove that regardless of the starting position, reading the digits in a clockwise direction will always result in a 1953-digit number that is divisible by 27.
(16th Moscow Mathematical Olympiad, 1953)

Solution

[Proof] Let the 1953-digit number starting from a certain position be
A=a1a2an1an(n=1953), A=\overline{a_{1} a_{2} \cdots a_{n-1} a_{n}} \quad(n=1953),

Then, by the given condition, 27A27 \mid A.
We only need to prove that B=ana1a2an1B=\overline{a_{n} a_{1} a_{2} \cdots a_{n-1}} is divisible by 27, and then, each time we move one digit from the end to the beginning, we can get all numbers divisible by 27.
Consider
10B+an 10 B+a_{n}
10B+an=an10n+A=an9991953+A+an. \begin{aligned} 10 B+a_{n} & =a_{n} \cdot 10^{n}+A \\ & =a_{n} \cdot \underbrace{99 \cdots 9}_{1953 \uparrow}+A+a_{n} . \end{aligned}

Since 99999 \cdots 9 and AA are both divisible by 27, then
1953
10B+anan(mod27),10B0(mod27). \begin{array}{l} 10 B+a_{n} \equiv a_{n} \quad(\bmod 27), \\ 10 B \equiv 0 \quad(\bmod 27) . \\ \end{array}

Also, (10,27)=1(10,27)=1, so B0(mod27)B \equiv 0 \quad(\bmod 27).
Thus, BB is divisible by 27.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.