Otherwise known as the much easier coin problem (BMT 2019 Discrete #2)
Source:
May 22, 2019
Problem Statement
Sylvia has a bag of 10 coins. Nine are fair coins, but the tenth has tails on both sides. Sylvia draws a coin at random from the bag and flips it without looking. If the coin comes up tails, what is the probability that the coin she drew was the 2-tailed coin?