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UKMT Grey Kangaroo, 2023

Combinatorics Difficulty 2.0 Junior Find the answer United Kingdom

Anna has five circular discs, each of a different size. She decides to build a tower using three of her discs so that each disc in her tower is smaller than the disc below it. How many different towers could Anna construct?

(A) $5$ (B) $6$ (C) $8$ (D) $10$ (E) $15$

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