The Game of Ur problem
Here’s a probability puzzle to ruin your week.
In the Royal Game of Ur, players advance tokens along a track with 14 spaces. To determine how many spaces to advance, a player rolls 4 dice with 4 sides. Two corners on each die are marked; the other two are not. The total number of marked corners — which is 0, 1, 2, 3, or 4 — is the number of spaces to advance.
For example, if the total on your first roll is 2, you could advance a token to space 2. If you roll a 3 on the next roll, you could advance the same token to space 5.
Suppose you have a token on space 13. How many rolls did it take to get there?
Hint: you might want to start by computing the distribution of k given n, where k is the number of the space and n is the number of rolls. Then think about the prior distribution of n.
I’ll post a solution later this week, but I have to confess: I believe my solution is correct, but there is still part of it I am not satisfied with.
[UPDATE November 1, 2018]
Here’s the thread on Twitter where a few people discuss this problem.
And here’s my solution. As you will see there are still some unresolved questions.
Here’s another solution from Austin Rochford, which estimates the posterior distribution by simulation.
And here’s a solution from vlad, also based on simulation, using WebPPL: