The probability of an event is equal to the number of favorable cases divided by the number of possible cases.
For example I do your question a).
a) Calculate the probability of getTing at least one die showing a 5 using two dices?
You can make a table to show all the possible and favorable cases. A event will be a par like
for example. As it is a 6x6 table, there are
possible cases. And if you count the favorable cases, you realize that there are
of them. So applying the simple formula I gave you, there is a probability of
to get at least a 5 pitching once two dices.