I was going to mention that. Also, if you want to compute the probability of just getting a yahtzee in 1's assuming you are only going for 1s as you did, then this result can be computed much more easily as:


[1-(5/6)^3]^5


That is, each die has up to 3 rolls to get a 1, so the chance of not getting it is (5/6)^3, the chance of getting it is 1-(5/6)^3, and getting it on all 5 dice is this raised to the 5th power. This should make the other computations easier as well.