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Combinatorics Difficulty 3.0 AMC 10/12 Find the answer United Kingdom

Mary had ten pieces of paper. Some of them were squares, and the rest were triangles. She cut three squares diagonally from corner to corner. She then found that the total number of vertices of the $13$ pieces of paper was $42$ . How many triangles did she have before making the cuts?

(A) $8$ (B) $7$ (C) $6$ (D) $5$ (E) $4$

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