I have been using this when interviewing people with math degrees:
Two players play a game with a single six-sided die. The player that starts can only win by rolling a 1. If he or she doesn't win, the other player gets to roll; he or she can only win by rolling a 6. The game continues until one player wins. What's the probability the first player wins (eventually)?
Chance to get a 1 (turn 1): 1/6
Chance to be allowed to roll: Previous chance to be allowed to roll * 5/6 (Chance player 1 didn't roll a 1) * 5/6 (Chance player 2 didn't roll a 6)
Chance to get a 1 (turns 2+): Chance to be allowed to roll * Chance to get a 1
I like this one. It can be brute-forced easily enough, but there are also a couple much more elegant approaches that I would expect a good mathematician to produce.
It tells me if they can at least minimally apply their knowledge and reason about a problem. It's a filter question. If a person with a college degree in math cannot solve this, what's the likelihood they will be able to solve an actual real problem I need them to work on?
Two players play a game with a single six-sided die. The player that starts can only win by rolling a 1. If he or she doesn't win, the other player gets to roll; he or she can only win by rolling a 6. The game continues until one player wins. What's the probability the first player wins (eventually)?