10 red marbles and 10 blue marbles are placed into a bag. Alex mixes up the bag and randomly selects a marble. He continues to do so, replacing the marble after each selection, until a red marble is selected. (10 points)
What is the probability that the first time that a red marble is pulled is on Alex’s 6th try?
On average, how many marbles will Alex have to pull in order to get a red marble? (Hint: use math expectation)