There’s a question in my Olympiad questions book which I can’t seem to solve:
You have the option to throw a die up to three times. You will earn
the face value of the die. You have the option to stop after each
throw and walk away with the money earned. The earnings are not additive. What is the expected payoff of this game?
I found a solution here but I don’t understand it.
Let’s suppose we have only 1 roll. What is the expected payoff? Each roll is equally likely, so it will show 1,2,3,4,5,6 with equal probability. Thus their average of 3.5 is the expected payoff.
Now let’s suppose we have 2 rolls. If on the first roll, I roll a 6, I would not continue. The next throw would only maintain my winnings of 6 (with 1/6 chance) or make me lose. Similarly, if I threw a 5 or a 4 on the first roll, I would not continue, because my expected payoff on the last throw would be a 3.5. However, if I threw a 1,2 of 3, I would take that second round. This is again because I expect to win 3.5.
So in the 2 roll game, if I roll a 4,5,6, I keep those rolls, but if I throw a 1,2,3, I reroll. Thus I have a 1/2 chance of keeping a 4,5,6, or a 1/2 chance of rerolling. Rerolling has an expected return of 3.5. As the 4,5,6 are equally likely, rolling a 4,5 or 6 has expected return 5. Thus my expected payout on 2 rolls is .5(5)+.5(3.5)=4.25.
Now we go to the 3 roll game. If I roll a 5 or 6, I keep my roll. But now, even a 4 is undesirable, because by rerolling, I’d be playing the 2 roll game, which has expected payout of 4.25. So now the expected payout is 13(5.5)+23(4.25)=4.¯66.
Does that make sense?