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The last whale erased

Source:

April 27, 2022

Problem Statement

Evan, Larry, and Alex are drawing whales on the whiteboard. Evan draws 10 whales, Larry draws 15 whales, and Alex draws 20 whales. Michelle then starts randomly erasing whales one by one. The probability that she finishes erasing Larry's whales first can be expressed as pq\frac{p}{q}. Compute p+qp+q.
2022 CCA Math Bonanza Team Round #10