MathDB
2020 BMT Discrete Tiebreaker #2

Source:

March 10, 2024
combinatorics

Problem Statement

On a certain planet, the alien inhabitants are born without any arms, legs, or noses. Every year, on their birthday, each alien randomly grows either an arm, a leg, or a nose, with equal probability for each. After its sixth birthday, the probability that an alien will have at least 22 arms, at least 22 legs, and at least 11 nose on the day is mn\frac{m}{n} , where mm and nn are relatively prime positive integers. Compute m+nm + n.