UKMT Grey Kangaroo, 2020

Combinatorics Difficulty 2.0 Junior Find the answer United Kingdom

The diagram shows a shape made up of 36 identical small equilateral triangles. What is the smallest number of small triangles identical to these that could be added to the shape to turn it into a hexagon?

(A) $10$ (B) $12$ (C) $15$ (D) $18$ (E) $24$

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