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MathNet 2026 — Part 08 Question 112

Math Olympiad Vault · MathNet 2026 — Part 08

Question 112

Let $M_{n}$ be the number of integers $N$ such that

(a) $0 \leq N \lt 10^{n}$;

(b) $N$ is divisible by $4$;

(c) The sum of the digits of $N$ is also divisible by $4$.
Prove that $M_{n} \neq 10^{n} / 16$ for all positive integers $n$.