Sorry, I goofed. This isn't quite right. It will give a rough number which should be pretty accurate, but it's not the exact probability. The reason is that I didn't account for the case of getting 2 of a kind on roll 1, and then the other 3 being a different 3 of a kind on roll 2, for example. Also, I counted twice when you get two pair, when each of these should only be counted once (22445, you can only count it as trying toward a 2-yahtzee or a 4-yahtzee, not both). The idea is the same, but I don't see any other way to find the exact probability other than enumerating all the possible cases in a decision tree and assigning probabilities in the way I did above. That's just a lot of number-crunching.