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Problem 449

Combinatorics Difficulty 2.0 Find the answer UKMT Pink Kangaroo · United Kingdom · 2016

Sven wrote five different one-digit positive integers on a blackboard. He discovered that none of the sums of two different numbers on the board equalled 10. Which of the following numbers did Sven definitely write on the blackboard?

(A) $1$ (B) $2$ (C) $3$ (D) $4$ (E) $5$

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