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UKMT Pink Kangaroo, 2017

Number theory Difficulty 3.0 AMC 10/12 Find the answer United Kingdom

How many positive integers $N$ possess the property that exactly one of the numbers $N$ and $(N+20)$ is a 4-digit number?

(A) $19$ (B) $20$ (C) $38$ (D) $39$ (E) $40$

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