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UKMT Junior Kangaroo, 2025

Combinatorics Difficulty 1.0 Junior Find the answer United Kingdom

Lily wants to make a large triangle from small triangular tiles. She has already put some tiles together, as shown in the diagram. What is the smallest number of small tiles that she now needs to complete a large triangle?

(A) $5$ (B) $9$ (C) $12$ (D) $15$ (E) $18$

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