My boy got this problem in Year 5:
Ben and Jenny roll a dice once. The first one to roll a five wins. What are the probability of winning?
The teacher gave an answer 1/6 for both players. I mentioned that whoever starts first has an upper hand as the second player doesn't have a chance to roll at all.
I asked Chat GPT, and the AI was confused at first. However, after a few pointers came up with a good reasoning when Ben starts first:
In this game, the players take turns rolling a dice with 6 sides. The first player to roll a 5 wins.
There are 3 possible outcomes in this game: Ben winning, Jenny winning, or a draw.
The probability of Ben winning is 1/6 on his first roll.
The probability of Jenny winning is the probability that Ben does not roll a 5 on his first roll, which is 5/6, multiplied by the probability that Jenny rolls a 5 on her first roll, which is also 1/6. Therefore, the probability of Jenny winning is (5/6) * (1/6) = 5/36.
The probability of a draw is the probability that neither player wins, which is the probability that both players roll a number other than 5. This can be calculated as (5/6) * (5/6) = 25/36.
As a bonus, what are the probabilities of winning if Ben and Jenny keep rolling until someone wins?
@Kail:
Yes, but the players will then not be competing against each other.
For example, you could say that both Ben and Jenny are playing against the house. Whoever rolls a 5 will win. But that is a fundamentally different game is what I'm trying to say.