Probability of B or C When I go to work. On day one, if my role is A, then on day two my role can be B or C, but not A. The roles are preditermined at the start of the month. On day two I used to think I have a 50% chance of being either role B or C. But lately I've thought  that is not accurate because my role is already predetermined, is it more acurate to say rather than a 50 % chance of B or C I have a 100 % chance of one of the roles and 0 % chance of the other? 
What caused me to think of this was the fact that if I arrive to work before my co worker. She says on her way that she has a 50% chance of B or C. But I have seen that she is on B. So I will say that she has a 100% chance of B and 0% chance of C. 
 A: This has to do what one means about probability. Often the idea of probability is assigning a number to the uncertainty that we have in life. So, when one says, "I have a $50\%$ chance having role $B$", one might mean, "Ignoring everything else, I expect to be assigned role $B$ with the same certainty of being assigned any other role". Another interpretation might be that you expect that in every situation that is similar to the one that you are now, if you calculate the percentage of times that you are assigned role $B$, then you will get that it will be $50%$ as you calculate the percentage for more and more days.
However, once you know what will happen, there is really no uncertainty (besides the small chance that there is a change or you get sick or something) and so it does not really apply.
It would be like me having performed very badly, so badly that I know that I will not get a promotion in almost perfect surety (I am in fact likely to be demoted), and the office management declares that it will promote someone in our office. So, I count the number of people in the office to get $5$ and declare that I have a $1/5 = 20\%$ chance of getting promoted. That number might be a good estimate if all of the people in the office were about equal in their work, ability, and all things relevant to being promoted, but in other cases it does not really apply.
That is at least my thoughts on your ideas.
