2024 SSMO Speed Round Problems/Problem 8

Problem

Bob has two coins; one is fair, and one lands on heads with a probability of $\frac{2}{3}.$ Bob chooses a random coin and flips it twice. Alice watches the two coin flips and guesses whether Bob flipped the fair or rigged coin. Given that Alice is a good mathematician and guesses the more likely option (guessing randomly when they are equally likely), the probability she guesses right can be expressed as $\frac{m}{n},$ for relatively prime positive integers \(m\) and \(n.\) Find $m+n.$

Solution